The coupled-cluster singles doubles model with perturbative treatment of triples (CCSD(T)) coupled with extrapolation to the complete basis-set limit and additive approaches represent the “golden standard” for the structural and spectroscopic characterization of building blocks of biomolecules and nanosystems. However, when open-shell systems are considered, additional problems related to both specific computational difficulties and the need of obtaining spin-dependent properties appear. In this contribution, we present a comprehensive study of the molecular structure and spectroscopic (IR, Raman, EPR) properties of the phenyl radical with the aim of validating an accurate computational protocol able to deal with conjugated open-shell species. We succeeded in obtaining reliable and accurate results, thus confirming and, partly, extending the available experimental data. The main issue to be pointed out is the need of going beyond the CCSD(T) level by including a full treatment of triple excitations in order to fulfil the accuracy requirements. On the other hand, the reliability of density functional theory in properly treating open-shell systems has been further confirmed.
Received 27 April 2013Accepted 24 May 2013Published online 19 June 2013
The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. ERC-2012-AdG-320951-DREAMS. This work was also supported by Italian MIUR (PRIN 2009, FIRB) and by the University of Bologna (RFO funds). The high performance computer facilities of the DREAMS center (http://dreamshpc.sns.it) are acknowledged for providing computer resources. The support of COST CMTS-Action CM1002 “COnvergent Distributed Environment for Computational Spectroscopy (CODECS)” is also acknowledged.
Article outline: I. INTRODUCTION II. METHODOLOGY A. Coupled-cluster calculations 1. Best-estimated molecular structure 2. Thermochemistry and molecular properties 3. Hyperfine couplings B. Evaluation of anharmonic contributions 1. Vibrational frequencies and zero-point vibrational energy 2. IR and Raman intensities 3. Vibrational averaging of molecular properties C. DFT and hybrid approaches 1. DFT calculations 2. Hybrid CC/DFT models III. RESULTS AND DISCUSSION A. Molecular structure and thermochemistry B. Electron spin resonance parameters C. IR and Raman spectra IV. CONCLUSIONS
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