Volume 141, Issue 20, 28 November 2014
Index of content:

Ice and water droplets on graphite have been studied by quantum path integral and classical molecular dynamics simulations. The pointcharge qTIP4P/F potential was used to model the interaction between flexible water molecules, while the watergraphite interaction was described by a LennardJones potential previously used to reproduce the macroscopic contact angle of water droplets on graphite. Several energetic and structural properties of water droplets with sizes between 10^{2} and 10^{3} molecules were analyzed in a temperature interval of 50–350 K. The vibrational density of states of crystalline and amorphous ice drops was correlated to the one of ice Ih to assess the influence of the droplet interface and molecular disorder on the vibrational properties. The average distance of covalent OH bonds is found 0.01 Å larger in the quantum limit than in the classical one. The OO distances are elongated by 0.03 Å in the quantum simulations at 50 K. Bond distance fluctuations are large as a consequence of the zeropoint vibrations. The analysis of the Hbond network shows that the liquid droplet is more structured in the classical limit than in the quantum case. The average kinetic and potential energy of the ice and water droplets on graphite has been compared with the values of ice Ih and liquid water as a function of temperature. The droplet kinetic energy shows a temperature dependence similar to the one of liquid water, without apparent discontinuity at temperatures where the droplet is solid. However, the droplet potential energy becomes significantly larger than the one of ice or water at the same temperature. In the quantum limit, the ice droplet is more expanded than in a classical description. Liquid droplets display identical density profiles and liquidvapor interfaces in the quantum and classical limits. The value of the contact angle is not influenced by quantum effects. Contact angles of droplets decrease as the size of the water droplet increases which implies a positive sign of the line tension of the droplet.
 COMMUNICATIONS


Communication: Estimating the initial biasing potential for λlocalelevation umbrellasampling (λLEUS) simulations via slow growth
View Description Hide DescriptionIn a recent article [Bieler et al. , J. Chem. Theory Comput.10, 3006–3022 (2014)], we introduced a combination of the λdynamics (λD) approach for calculating alchemical freeenergy differences and of the localelevation umbrellasampling (LEUS) memorybased biasing method to enhance the sampling along the alchemical coordinate. The combined scheme, referred to as λLEUS, was applied to the perturbation of hydroquinone to benzene in water as a test system, and found to represent an improvement over thermodynamic integration (TI) in terms of sampling efficiency at equivalent accuracy. However, the preoptimization of the biasing potential required in the λLEUS method requires “filling up” all the basins in the potential of mean force. This introduces a nonproductive presampling time that is systemdependent, and generally exceeds the corresponding equilibration time in a TI calculation. In this letter, a remedy is proposed to this problem, termed the slow growth memory guessing (SGMG) approach. Instead of initializing the biasing potential to zero at the start of the preoptimization, an approximate potential of mean force is estimated from a short slow growth calculation, and its negative used to construct the initial memory. Considering the same test system as in the preceding article, it is shown that of the application of SGMG in λLEUS permits to reduce the preoptimization time by about a factor of four.

 ARTICLES

 Theoretical Methods and Algorithms

Towards improved local hybrid functionals by calibration of exchangeenergy densities
View Description Hide DescriptionA new approach for the calibration of (semi)local and exact exchangeenergy densities in the context of local hybrid functionals is reported. The calibration functions are derived from only the electron density and its spatial derivatives, avoiding spatial derivatives of the exactexchange energy density or other computationally unfavorable contributions. The calibration functions fulfill the seven more important out of nine known exact constraints. It is shown that calibration improves substantially the definition of a nondynamical correlation energy term for generalized gradient approximation (GGA)based local hybrids. Moreover, gauge artifacts in the potentialenergy curves of noblegas dimers may be corrected by calibration. The developed calibration functions are then evaluated for a large range of energyrelated properties (atomization energies, reaction barriers, ionization potentials, electron affinities, and total atomic energies) of three sets of local hybrids, using a simple oneparameter localmixing. The functionals are based on (a) local spindensity approximation (LSDA) or (b) PerdewBurkeErnzerhof (PBE) exchange and correlation, and on (c) Becke88 (B88) exchange and LeeYangParr (LYP) correlation. While the uncalibrated GGAbased functionals usually provide very poor thermochemical data, calibration allows a dramatic improvement, accompanied by only a small deterioration of reaction barriers. In particular, an optimized BLYPbased localhybrid functional has been found that is a substantial improvement over the underlying global hybrids, as well as over previously reported LSDAbased local hybrids. It is expected that the present calibration approach will pave the way towards new generations of more accurate hyperGGA functionals based on a local mixing of exchangeenergy densities.

A multiblob approach to colloidal hydrodynamics with inherent lubrication
View Description Hide DescriptionThis work presents an intermediate resolution model of the hydrodynamics of colloidal particles based on a mixed EulerianLagrangian formulation. The particle is constructed with a small set of overlapping Peskin's Immersed Boundary kernels (blobs) which are held together by springs to build up a particle impenetrable core. Here, we used 12 blobs placed in the vertexes of an icosahedron with an extra one in its center. Although the particle surface is not explicitly resolved, we show that the shortdistance hydrodynamic responses (flow profiles, translational and rotational mobilities) agree with spherical colloids and provide consistent effective radii. A remarkable property of the present multiblob model is that it naturally provides zero relative mobility at some finite interparticle distance. In terms of mutual friction, this divergent force accurately represents the “soft” lubrication regime of spherical colloids and permits to resolve the increase of the solution viscosity up to moderately dense systems with volume fraction up to about 0.50. This intermediate resolution model is able to recover highly nontrivial (manybody) hydrodynamics using small particles whose radii are similar to the grid size h (in the range [1.6 − 3.2] h). Considering that the cost of the embedding fluid phase scales such as the cube of the particle radius, this result brings about a significant computational speedup. Our code Fluam works in Graphics Processor Units and uses Fast Fourier Transform for the Poisson solver, which further improves its efficiency.

Rotational averaging of multiphoton absorption cross sections
View Description Hide DescriptionRotational averaging of tensors is a crucial step in the calculation of molecular properties in isotropic media. We present a scheme for the rotational averaging of multiphoton absorption cross sections. We extend existing literature on rotational averaging to evenrank tensors of arbitrary order and derive equations that require only the number of photons as input. In particular, we derive the first explicit expressions for the rotational average of five, six, and sevenphoton absorption cross sections. This work is one of the required steps in making the calculation of these higherorder absorption properties possible. The results can be applied to any evenrank tensor provided linearly polarized light is used.

On the time scale associated with Monte Carlo simulations
View Description Hide DescriptionUniformacceptance forcebias Monte Carlo (fbMC) methods have been shown to be a powerful technique to access longer timescales in atomistic simulations allowing, for example, phase transitions and growth. Recently, a new fbMC method, the timestamped forcebias Monte Carlo (tfMC) method, was derived with inclusion of an estimated effective timescale; this timescale, however, does not seem able to explain some of the successes the method. In this contribution, we therefore explicitly quantify the effective timescale tfMC is able to access for a variety of systems, namely a simple singleparticle, onedimensional model system, the LennardJones liquid, an adatom on the Cu(100) surface, a silicon crystal with point defects and a highly defected graphene sheet, in order to gain new insights into the mechanisms by which tfMC operates. It is found that considerable boosts, up to three orders of magnitude compared to molecular dynamics, can be achieved for solid state systems by lowering of the apparent activation barrier of occurring processes, while not requiring any systemspecific input or modifications of the method. We furthermore address the pitfalls of using the method as a replacement or complement of molecular dynamics simulations, its ability to explicitly describe correct dynamics and reaction mechanisms, and the association of timescales to MC simulations in general.

Orbitaloptimized MP2.5 and its analytic gradients: Approaching CCSD(T) quality for noncovalent interactions
View Description Hide DescriptionOrbitaloptimized MP2.5 [or simply “optimized MP2.5,” OMP2.5, for short] and its analytic energy gradients are presented. The cost of the presented method is as much as that of coupledcluster singles and doubles (CCSD) [O(N ^{6}) scaling] for energy computations. However, for analytic gradient computations the OMP2.5 method is only half as expensive as CCSD because there is no need to solve λ2amplitude equations for OMP2.5. The performance of the OMP2.5 method is compared with that of the standard secondorder Møller–Plesset perturbation theory (MP2), MP2.5, CCSD, and coupledcluster singles and doubles with perturbative triples (CCSD(T)) methods for equilibrium geometries, hydrogen transfer reactions between radicals, and noncovalent interactions. For bond lengths of both closed and openshell molecules, the OMP2.5 method improves upon MP2.5 and CCSD by 38%–43% and 31%–28%, respectively, with Dunning's ccpCVQZ basis set. For complete basis set (CBS) predictions of hydrogen transfer reaction energies, the OMP2.5 method exhibits a substantially better performance than MP2.5, providing a mean absolute error of 1.1 kcal mol^{−1}, which is more than 10 times lower than that of MP2.5 (11.8 kcal mol^{−1}), and comparing to MP2 (14.6 kcal mol^{−1}) there is a more than 12fold reduction in errors. For noncovalent interaction energies (at CBS limits), the OMP2.5 method maintains the very good performance of MP2.5 for closedshell systems, and for openshell systems it significantly outperforms MP2.5 and CCSD, and approaches CCSD(T) quality. The MP2.5 errors decrease by a factor of 5 when the optimized orbitals are used for openshell noncovalent interactions, and comparing to CCSD there is a more than 3fold reduction in errors. Overall, the present application results indicate that the OMP2.5 method is very promising for openshell noncovalent interactions and other chemical systems with difficult electronic structures.

Multidimensional reaction rate theory with anisotropic diffusion
View Description Hide DescriptionAn analytical expression is derived for the rate constant that describes diffusive transitions between two deep wells of a multidimensional potential. The expression, in contrast to the KramersLanger formula for the rate constant, is valid even when the diffusion is highly anisotropic. Our approach is based on a variational principle for the reactive flux and uses a trial function for the splitting probability or commitor. The theoretical result is validated by Brownian dynamics simulations.

Stability conditions for exactexchange KohnSham methods and their relation to correlation energies from the adiabaticconnection fluctuationdissipation theorem
View Description Hide DescriptionThe occurrence of instabilities, in particular singlettriplet and singletsinglet instabilities, in the exactexchange (EXX) KohnSham method is investigated. Hessian matrices of the EXX electronic energy with respect to the expansion coefficients of the EXX effective KohnSham potential in an auxiliary basis set are derived. The eigenvalues of these Hessian matrices determine whether or not instabilities are present. Similar as in the corresponding HartreeFock case instabilities in the EXX method are related to symmetry breaking of the Hamiltonian operator for the EXX orbitals. In the EXX methods symmetry breaking can easily be visualized by displaying the local multiplicative exchange potential. Examples (N2, O2, and the polyyne C10H2) for instabilities and symmetry breaking are discussed. The relation of the stability conditions for EXX methods to approaches calculating the KohnSham correlation energy via the adiabaticconnection fluctuationdissipation (ACFD) theorem is discussed. The existence or nonexistence of singletsinglet instabilities in an EXX calculation is shown to indicate whether or not the frequencyintegration in the evaluation of the correlation energy is singular in the EXXACFD method. This method calculates the KohnSham correlation energy through the ACFD theorem theorem employing besides the Coulomb kernel also the full frequencydependent exchange kernel and yields highly accurate electronic energies. For the case of singular frequencyintegrands in the EXXACFD method a regularization is suggested. Finally, we present examples of molecular systems for which the selfconsistent field procedure of the EXX as well as the HartreeFock method can converge to more than one local minimum depending on the initial conditions.

Exponential approximation for onecomponent Yukawa plasma
View Description Hide DescriptionA theory based on the exponential approximation of the liquidstate theory is applied to study properties of several models of onecomponent Yukawa plasma characterized by different values of the screening parameter z. The results of the new theory are compared to the results of a conventional theory, which is based on the firstorder mean spherical approximation, and to the results of a Monte Carlo simulation. The new theory shows improvements in the predictions for the thermodynamic and structural properties of Yukawa plasmas with high and intermediate values of the screening parameter, z, and coupling parameter, Γ. For low values of z and Γ, the new theory is comparable in accuracy to the conventional theory, which in turn agrees well with the results of the Monte Carlo simulation.

Lazy Updating of hubs can enable more realistic models by speeding up stochastic simulations
View Description Hide DescriptionTo respect the nature of discrete parts in a system, stochastic simulation algorithms (SSAs) must update for each action (i) all part counts and (ii) each action's probability of occurring next and its timing. This makes it expensive to simulate biological networks with wellconnected “hubs” such as ATP that affect many actions. Temperature and volume also affect many actions and may be changed significantly in small steps by the network itself during fever and cell growth, respectively. Such trends matter for evolutionary questions, as cell volume determines doubling times and fever may affect survival, both key traits for biological evolution. Yet simulations often ignore such trends and assume constant environments to avoid many costly probability updates. Such computational convenience precludes analyses of important aspects of evolution. Here we present “Lazy Updating,” an addon for SSAs designed to reduce the cost of simulating hubs. When a hub changes, Lazy Updating postpones all probability updates for reactions depending on this hub, until a threshold is crossed. Speedup is substantial if most computing time is spent on such updates. We implemented Lazy Updating for the Sorting Direct Method and it is easily integrated into other SSAs such as Gillespie's Direct Method or the Next Reaction Method. Testing on several toy models and a cellular metabolism model showed >10× faster simulations for its usecases—with a small loss of accuracy. Thus we see Lazy Updating as a valuable tool for some special but important simulation problems that are difficult to address efficiently otherwise.
 Atoms, Molecules, and Clusters

Pulsetrain control of photofragmentation at constant field energy
View Description Hide DescriptionWe consider a phaselocked twopulse sequence applied to photofragmentation in the weakfield limit. The two pulses are not overlapping in time, i.e., the energy of the pulsetrain is constant for all time delays. It is shown that the relative yield of excited in the nonadiabatic process: I + Br* ← IBr → I + Br, changes as a function of time delay when the two excited wave packets interfere. The underlying mechanisms are analyzed and the change in the branching ratio as a function of time delay is only a reflection of a changing frequency distribution of the pulse train; the branching ratio does not depend on the detailed pulse shape.

The electronic structure of vanadium monochloride cation (VCl^{ + }): Tackling the complexities of transition metal species
View Description Hide DescriptionSix electronic states (X ^{4}Σ^{−}, A ^{4}Π, B ^{4}Δ, ^{2}Φ, ^{2}Δ, ^{2}Σ^{+}) of the vanadium monochloride cation (VCl^{+}) are described using large basis set coupled cluster theory. For the two lowest quartet states (X ^{4}Σ^{−} and A ^{4}Π), a focal point analysis (FPA) approach was used that conjoined a correlationconsistent family of basis sets up to augccpwCV5ZDK with highorder coupled cluster theory through pentuple (CCSDTQP) excitations. FPA adiabatic excitation energies (T 0) and spectroscopic constants (r e, r 0, B e, B 0, e, H e, ω e, v 0, α e, ω e x e) were extrapolated to the valence complete basis set DouglasKroll (DK) augccpV∞ZDK CCSDT level of theory, and additional treatments accounted for higherorder valence electron correlation, core correlation, and spinorbit coupling. Due to the delicate interplay between dynamical and static electronic correlation, single reference coupled cluster theory is able to provide the correct ground electronic state (X ^{4}Σ^{−}), while multireference configuration interaction theory cannot. Perturbations from the first and secondorder spin orbit coupling of lowlying states with quartet spin multiplicity reveal an immensely complex rotational spectrum relative to the isovalent species VO, VS, and TiCl. Computational data on the doublet manifold suggest that the lowestlying doublet state (^{2}Γ) has a T e of ∼11 200 cm^{−1}. Overall, this study shows that laboratory and theoretical rotational spectroscopists must work more closely in tandem to better understand the bonding and structure of molecules containing transition metals.

Laserdriven localization of collective CO vibrations in metalcarbonyl complexes
View Description Hide DescriptionUsing the example of a cobalt dicarbonyl complex it is shown that two perpendicular linearly polarized IR laser pulses can be used to trigger an excitation of the delocalized CO stretching modes, which corresponds to an alternating localization of the vibration within one CO bond. The switching time for localization in either of the two bonds is determined by the energy gap between the symmetric and asymmetric fundamental transition frequencies. The phase of the oscillation between the two local bond excitations can be tuned by the relative phase of the two pulses. The extend of control of bond localization is limited by the anharmonicity of the potential energy surfaces leading to wave packet dispersion. This prevents such a simple pulse scheme from being used for laserdriven bond breaking in the considered example.

Reactions of liquid and solid aluminum clusters with N2: The role of structure and phase in Al114 ^{+}, Al115 ^{+}, and Al117 ^{+}
View Description Hide DescriptionKinetic energy thresholds have been measured for the chemisorption of N2 onto Al 114 ^{+}, Al 115 ^{+}, and Al 117 ^{+} as a function of the cluster's initial temperature, from around 200 K up to around 900 K. For all three clusters there is a sharp drop in the kinetic energy threshold of 0.5–0.6 eV at around 450 K, that is correlated with the structural transition identified in heat capacity measurements. The decrease in the thresholds corresponds to an increase in the reaction rate constant, k(T) at 450 K, of around 10^{6}fold. No significant change in the thresholds occurs when the clusters melt at around 600 K. This contrasts with behavior previously reported for smaller clusters where a substantial drop in the kinetic energy thresholds is correlated with the melting transition.

Near ultraviolet photodissociation spectroscopy of Mn^{+}(H2O) and Mn^{+}(D2O)
View Description Hide DescriptionThe electronic spectra of Mn^{+}(H2O) and Mn^{+}(D2O) have been measured from 30 000 to 35 000 cm^{−1} using photodissociation spectroscopy. Transitions are observed from the ^{7}A1 ground state in which the Mn^{+} is in a 3d ^{5}4s ^{1} electronic configuration, to the ^{7}B2 (3d ^{5}4p y) and ^{7}B1 (3d ^{5}4p x) excited states with T0 = 30 210 and 32 274 cm^{−1}, respectively. Each electronic transition has partially resolved rotational and extensive vibrational structure with an extended progression in the metal−ligand stretch at a frequency of ∼450 cm^{−1}. There are also progressions in the inplane bend in the ^{7}B2 state, due to vibronic coupling, and the outofplane bend in the ^{7}B1 state, where the calculation illustrates that this state is slightly nonplanar. Electronic structure computations at the CCSD(T)/augccpVTZ and TDDFT B3LYP/6311++G(3df,3pd) level are also used to characterize the ground and excited states, respectively. These calculations predict a ground state MnO bond length of 2.18 Å. Analysis of the experimentally observed vibrational intensities reveals that this bond length decreases by 0.15 ± 0.015 Å and 0.14 ± 0.01 Å in the excited states. The behavior is accounted for by the less repulsive px and py orbitals causing the Mn^{+} to interact more strongly with water in the excited states than the ground state. The result is a decrease in the MnO bond length, along with an increase in the HOH angle. The spectra have well resolved K rotational structure. Fitting this structure gives spinrotation constants ɛaa″ = −3 ± 1 cm^{−1} for the ground state and ɛaa′ = 0.5 ± 0.5 cm^{−1} and εaa′ = −4.2 ± 0.7 cm^{−1} for the first and second excited states, respectively, and A′ = 12.8 ± 0.7 cm^{−1} for the first excited state. Vibrationally mediated photodissociation studies determine the OH antisymmetric stretching frequency in the ground electronic state to be 3658 cm^{−1}.

Conical intersections and diabatic potential energy surfaces for the three lowest electronic singlet states of
View Description Hide DescriptionWe calculate the adiabatic Potential Energy Surfaces (PESs) and the NonAdiabatic Coupling Terms (NACTs) for the three lowest singlet states of in hyperspherical coordinates as functions of hyperangles (θ and ϕ) for a grid of fixed values of hyperradius (1.5 ⩽ ρ ⩽ 20 bohrs) using the MRCI level of methodology employing ab initio quantum chemistry package (MOLPRO). The NACT between the ground and the first excited state translates along the seams on the θ − ϕ space, i.e., there are six Conical Intersections (CIs) at each θ (60° ⩽ θ ⩽ 90°) within the domain, 0 ⩽ ϕ ⩽ 2π. While transforming the adiabatic PESs to the diabatic ones, such surfaces show up six crossings along those seams. Our beyond BornOppenheimer approach could incorporate the effect of NACTs accurately and construct singlevalued, continuous, smooth, and symmetric diabatic PESs. Since the location of CIs and the spatial amplitudes of NACTs are most prominent around ρ = 10 bohrs, generally only those results are depicted.

The vibrational dependence of dissociative recombination: Rate constants for
View Description Hide DescriptionDissociative recombination rate constants are reported with electron temperature dependent uncertainties for the lowest 5 vibrational levels of the ground state. The rate constants are determined from ab initio calculations of potential curves, electronic widths, quantum defects, and cross sections. At 100 K electron temperature, the rate constants overlap with the exception of the third vibrational level. At and above 300 K, the rate constants for excited vibrational levels are significantly smaller than that for the ground level. It is shown that any experimentally determined total rate constant at 300 K electron temperature that is smaller than 2.0 × 10^{−7} cm^{3}/s is likely to be for ions that have a substantially excited vibrational population. Using the vibrational level specific rate constants, the total rate constant is in very good agreement with that for an excited vibrational distribution found in a storage ring experiment. It is also shown that a prior analysis of a laser induced fluorescence experiment is quantitatively flawed due to the need to account for reactions with unknown rate constants. Two prior calculations of the dissociative recombination rate constant are shown to be inconsistent with the cross sections upon which they are based. The rate constants calculated here contribute to the resolution of a 30 year old disagreement between modeled and observed ionospheric densities.

The effects of methyl internal rotation and ^{14}N quadrupole coupling in the microwave spectra of two conformers of N,Ndiethylacetamide
View Description Hide DescriptionThe gas phase structures and internal dynamics of N,Ndiethylacetamide were determined with very high accuracy using a combination of molecular beam Fouriertransform microwave spectroscopy and quantum chemical calculations at high levels. Conformational studies yielded five stable conformers with C1 symmetry. The two most energetically favorable conformers, conformer I and II, could be found in the experimental spectrum. For both conformers, quadrupole hyperfine splittings of the ^{14}N nucleus and torsional fine splittings due to the internal rotation of the acetyl methyl group occurred in the same order of magnitude and were fully assigned. The rotational constants, centrifugal distortion constants as well as the quadrupole coupling constants of the ^{14}N nucleus were determined and fitted to experimental accuracy. The V 3 potentials were found to be 517.04(13) cm^{−1} and 619.48(91) cm^{−1} for conformer I and II, respectively, and compared to the V 3 potentials found in other acetamides. Highly accurate CCSD(T) and DMC calculations were carried out for calculating the barriers to internal rotation in comparison with the experimentally deduced V 3 values.

Controversial electronic structures and energies of Fe2, , and resolved by RASPT2 calculations
View Description Hide DescriptionThe diatomic molecule Fe2 was investigated using restricted active space secondorder perturbation theory (RASPT2). This molecule is very challenging to study computationally because predictions about the ground state and excited states depend sensitively on the choice of the quantum chemical method. For Fe2 we show that one needs to go beyond a fullvalence active space in order to achieve even qualitative agreement with experiment for the dissociation energy, and we also obtain a smooth groundstate potential curve. In addition we report the first multireference study of , for which we predict an ^{8} ground state, which was not predicted by previous computational studies. By using an active space large enough to remove the most serious deficiencies of previous theoretical work and by explicitly investigating the interpretations of previous experimental results, this study elucidates previous difficulties and provides – for the first time – a qualitatively correct treatment of Fe2, , and . Moreover, this study represents a record in terms of the number or active electrons and active orbitals in the active space, namely 16 electrons in 28 orbitals. Conventional CASPT2 calculations can be performed with at most 16 electrons in 16 orbitals. We were able to overcome this limit by using the RASPT2 formalism.

Reaction of the C3(X^{1}Σg ^{+}) carbon cluster with H2S(X^{1}A1), hydrogen sulfide: Photoninduced formation of C3S, tricarbon sulfur
View Description Hide DescriptionIn this paper we report on the neutralneutral reaction of the C3 carbon cluster with H2S in solid inert argon at 12 K, conditions that mimic, in part, the surfaces of interstellar grains. In the first step of the reaction, a C3•H2S complex is formed via an almost barrierless entrance addition mechanism. This complex, stabilized by an estimated 7.45 kJ/mol (CCSD(T)/augccpVTZ//B3LYP/6311++G(d,p) level), is formed by the interaction of a terminal carbon of C3 with a hydrogen in H2S. This concovalent complex displays a band at 2044.1 cm^{−1} observed via Fourier transform infrared absorption spectroscopy. With the help of the MP2/augccpVDZ level method, this band is assigned to the CC asymmetric vibration mode. When the complex is exposed to UVvisible photons (hν < 5.5 eV) the tricarbon sulfur C3S molecule is identified, based on the appearance of a characteristic CC stretching band at 2047.5 cm^{−1}. Calculated groundstate potential energy surfaces also confirm the concomitant formation of molecular H2. This facile reaction pathway involves an attainable transition state of 174.4 kJ/mol. Conversely, competing lowerenergy reaction pathways that would lead to the generation of H2C3S (propadienethione), or C2H2 (acetylene) and CS, involve much more complex, multistage pathways, and are not observed experimentally.