58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

This project concerns the development of new massively parallel coupled-cluster (CC) implementations to accurately predict electronic spin resonance (ESR) tensors in large radicals. Free radicals play a crucial role in many biological and industrial processes, particularly in petroleum industry.  ESR spectroscopy is a technique that can uniquely study the nature of free radicals alone and also in their cooperative effect during chemical reactions. One of the difficulties with ESR spectra is that they are far more complex to interpret than those from other widely used spectroscopic techniques, such as nuclear magnetic resonance and infrared spectroscopies. Therefore, being able to simulate ESR spectra theoretically is an invaluable and indispensable component in their analysis. However, in order to be useful for such analysis, the employed theory must be absolutely predictive in the sense that it can be reliably used as a complement to experiments. Density functional theory (DFT) models have been widely applied to ESR simulations for large systems of biological and industrial interests, but they have been proven to be unreliable and fraught with many difficulties. On the other hand, more accurate wavefunction-based methods with predictive quality remain limited to small benchmark systems. In particular, CC methods are known to provide the most accurate results for nearly all the electronic structure problems including ESR spectra. In fact, one of the CC models, commonly designated as CCSD(T), is considered as the gold standard for electronic structure calculations by the quantum chemistry community. However, the high computational cost of the CC methods has precluded their routine application to large systems. To overcome this serious limitation, we are developing massively parallel coupled-cluster (CC) implementations to predict ESR properties. Those implementations follow a new domain specific software paradigm developed by the Bartlett research group for its public domain code ACES III. This paradigm includes the super instruction processor (SIP) and the super instruction architecture language (SIAL) to run and develop highly efficient parallel codes. In that framework, we are further extending the massively parallel CC implementations of ACES III to systematically predict the three main ESR tensors in large radicals. Those tensors are: the hyperfine coupling A-tensor, which quantifies the interaction between nuclear and electronic spin angular momenta; the g-tensor, which quantifies the interaction between electronic spin and orbital angular momenta; and the D-tensor (also known as the zero-field splitting tensor), which quantifies electronic spin interactions in systems having more than one unpaired electron. During the first reporting period of this grant, we started the derivation and implementation of the first two ESR tensors, the A- and g-tensors. At present, the capabilities to calculate the A-tensor have been completely implemented in ACES III and applied to various organic radicals. Results of this completed first stage of the project have been reported in an article submitted for publication to the Journal of Chemical Physics. That article is currently under review at the minor revision stage. Those reported massively parallel A-tensor capabilities have been applied to calculate the isotropic hyperfine coupling constants in 38 neutral, cationic, and anionic radicals that include the B, O, Be, F, H, C, Cl, S, N, P, and Zn nuclei. Some of the large radicals under study included: diethylaminyl radical, benzyl radical, 1,3,2-benzodithiazolyl radical,  phenylaminyl radical, cyclo-hexyl radical, aniline cation radical, 4-nitroaniline cation radical, p-phenylenediamine cation radical, N,N-dimethylamino-p-phenylenediamine cation radical , N,N-Dimethyl-4-nitroaniline cation radical, 1-adamantyl radical , and Zn-porphycene anion radical. The reported parallel calculations were conducted at the Hartree-Fock (HF), second-order many-body perturbation theory [MBPT(2)], CC singles and doubles (CCSD), and CCSD with perturbative triples [CCSD(T)] levels using various large atomic basis sets. HF results consistently overestimated isotropic hyperfine coupling constants. However, inclusion of electron correlation effects in the simplest way via MBPT(2) provided significant improvements in the predictions, but not without occasional failures. In contrast, CCSD and CCSD(T) results were consistently in very good agreement with experimental results. The reported hyperfine tensor calculations for the above radicals with up to 35 atoms and with up to 925 basis functions are among the largest applications of the CC theory in general and constitute the largest CC prediction of ESR spectra to date. Currently, we are making progress in the derivation and implementation of the g-tensors capabilities in ACES III, which will be completed soon. A paper reporting that effort and its application to large radicals is expected to be submitted for publication in a few months.