Improving Dispersion Energy Calculations

Abstract

Various approaches were investigated and developed to improve dispersion energy calculations. The highest levels of ab initio electronic structure theory available were applied to study nine characteristic points on the CO–CO, N2–N2, and CO–N2 potential energy surfaces. To obtain the desired spectroscopic accuracy, corrections beyond the state-of-the-art CCSD(T)/CBS level of theory were calculated, including higher-orders of coupled-cluster theory (up to full single, double, triple, and quadruple excitations, CCSDTQ), relativistic effects, and core-core and core-valence correlation. The significance of post-CCSD(T) effects was emphasized. These effects are particularly important for the CO–CO complex: their magnitude can exceed 3% of the CCSD(T) interaction energy. In order to enhance the dispersion calculations for the D3 atom-pairwise dispersion correction by Grimme, new forms of damping were designed: a linear combination of error functions and a piecewise-linear function. Furthermore, the possibility of creating a damped dispersion function without higher than C6 dispersion coefficients was explored. Last but not least, second-order dispersion and exchange-dispersion corrections in SAPT were improved by applying the F12 methods, resulting in E(20)disp-F12 and E(20)exch-disp-F12. In order to improve scaling of these methods, we proposed three approximate Ansätze: EBC, optimized diagonal, and fixed-amplitude. Moreover, the density-fitting algorithm was introduced to speed up the calculations and to overcome the memory bottleneck.