Natural orbitals (NOs) and the occupation patterns of the dominant electronic configurations (with weights of approximately 68% and 77%, respectively) for the electronic ground state and its high-spin counterpart state of FeCO, calculated at the CASSCF level of theory under C 2v symmetry constraint. The NOs are schematically represented in terms of Fe 3d, 4s, and 4p atomic orbitals (AOs) and C and O 2s and 2p AOs. The ordinate represents the relative energies of the NOs, estimated as the diagonal elements of the effective Fock matrix in the NO basis (see Sec. II).
The 10σ, 4π, and 12σ NOs of and MR-SDCI wavefunctions expressed in terms of the iso-surface with orbital amplitude ±0.02 a 0 −3/2. For each NO, the occupation number is given in parentheses.
The process of Fe–C bond formation in FeCO with r(C–O) fixed at r e(C–O) = 1.1587 Å. The abscissa is r(Fe–C), the distance between Fe and C, and r e(Fe–C) is indicated by a vertical dashed line. The NO levels (c) and their occupation numbers (d) are those of the CASSCF wavefunction. The AO coefficients in (d) and (e) are calculated by Eq. (1) from the associated primitive basis-function coefficients, with the sign of the largest component, in an NO of the MR-SDCI wavefunction.
Experimentally and ab initio derived molecular constants for FeCO. See footnotes for the ω1(C–O), ω2(bend), and ω3(Fe–C) values available.
Electronic term values T e() (in cm−1), bond lengths r e (in Å), and dipole moments μe (in D) of and FeCO for each equilibrium geometry calculated for various reference spaces (given for each method in the first line of the respective entry) in the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] calculation under C 2v symmetry constraint. The reference space for the [0-0-0-1] CASSCF NOs employed in the MR-SDCI+Q_DK3 calculation is “occ,14,5,5,1; frozen,0,0,0,0; closed,7,2,2,0”; it is common to the methods (1)–(4), and also to the [0-0-0-1] CASSCF calculation in method (0).
Equilibrium geometries and energies, and some molecular constants at each equilibrium geometry, of the ground and low-lying excited states of FeCO, calculated under C 2v symmetry at the MR-SDCI+Q_DK3/[5ZP ANO-RCC (Fe, C, O)] or MR-AQCC_DK3/[5ZP ANO-RCC (Fe, C, O)] level of theory. The equilibrium bond angle (Fe–C–O) is checked by separate calculations to be 180° for all states considered with the exception of 5Δ.
Vertical excitation energies and dipole moments of various states at the and MR-SDCI+Q_DK3 equilibrium geometries, calculated at the MR-AQCC_DK3/[5ZP ANO-RCC (Fe, C, O)] level of theory.
Dynamical electron correlation energies for the low-spin and high-spin state pair of FeCO, and comparison of the properties of these pair states.
AO coefficients, defined as in Eq. (1), in the NOs of the and CASSCF wavefunctions at each equilibrium geometry.a
AO coefficients, defined as in Eq. (1), in the NOs of the and MR-SDCI wavefunctions at each equilibrium geometry.a
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