Complete basis set extrapolation and hybrid schemes for geometry gradients and vibrational analysis of noncovalent complexes

Jiří Černý

Institute of Biotechnology, Academy of Sciences of the Czech Republic, v. v. i., Vídeňská 1083, 142 20, Praha 4, Czech Republic

This work focuses on the performance of popular WFT (MP2, MP2.5, MP3, SCS(MI)-MP2, CCSD(T)) and DFT (M06-2X, TPSS-D) methods in optimizations of geometries of noncovalent complexes. Apart from the straightforward comparison of the accuracy of the resulting geometries with respect to the most accurate, computationally affordable, reference method, we have also attempted to determine the most efficient utilization of the information contained in the gradient of a particular method and basis set. Essentially, we have transfered the ideas successfully used for noncovalent interaction energy calculations to geometry optimizations.[1]

We have found that CCSD(T) geometries are most faithfully reproduced by the MP2.5 and MP3 methods, followed by the comparably well performing SCS(MI)-MP2 and MP2 methods, finally by the worst performing DFT-D and M06 methods.

Basis set extrapolation of gradients was shown to improve the results and can be considered as a low-cost alternative to the use of CP-corrected gradients. A hybrid gradient scheme was shown to deliver geometries close to the regular gradient reference. Analogously to a similar hybrid scheme, which nowadays is routinely used for the calculation of interaction energies, such a hybrid gradient scheme can save a huge amount of computer time, when high accuracy is desired.

We will also present further extension of the highly accurate hybrid and CBS extrapolated gradients schemes for calculation of numerical vibrational frequencies of isolated molecules as well as their noncovalent complexes.

[1] J. Černý*, M. Pitoňák, K. E. Riley, P. Hobza, J. Chem. Theory Comput., 2011, 7 (12), 3924-3934