ABSTRACT:In this work, we investigate the correlation between error introduced by truncation of optimized virtual orbital space (OVOS) on the MP2 level (Y MP2 ) with the error of the post-MP2 contributions, such as the CCSD-MP2 (i.e., Y CCSD ), CCSD(T)-MP2 (i.e., Y CCSD(T) ), or the (T) separately. We found a correlation between the Y MP2 and several other quantities, such as the percent of recovered optimization functional value in the
HIGHLY CORRELATED CALCULATIONS USING OVOS truncated OVOS (OF%), or the aforementioned Y CCSD , Y CCSD(T) , Y (T), which is to good approximation linear in the logarithmic scale. These correlations open a possibility to control the accuracy of the post-MP2 calculations in the truncated OVOS, because the Y MP2 and the OF% are easily obtained, almost as a byproduct of the virtual orbital optimization. According to the results present in this work, knowledge of the Y MP2 or the OF% allows us to safely estimate the order of magnitude of the error of the post-MP2 corrections. To keep the accuracy of, for instance, CCSD(T) correlation energy calculated in the truncated OVOS within 1.10 −5 − 1.10 −6 Hartree error bars, we can typically reduce only a few percent of the OVOS, although this value increases slightly with enlarging the atomic orbital (AO) basis set and the number of inactive occupied orbitals. Still, even such a modest reduction can save more then a half of the computation time, compared with calculations in the full VOS. The situation is, however, much more favourable in case of counterpoise (CP) corrected calculations of interaction energy, where this methodology enables safe truncation of significant part of OVOS of monomers, resulting from the presence of the ghost AO. Dimension of such truncated OVOS essentially corresponds to CP-uncorrected calculations, thus leads to more than an order of magnitude speedup of calculation of monomers.
