Analytic gradients for the multicentred integrated QM:QM method for weakly bound clusters: efficient and accurate 2-body:many-body geometry optimizations

Abstract

In this work we present analytic gradients for the multicentred integrated QM:QM method for weakly bound clusters that was recently introduced [Chem. Phys. Lett. 427,185 (2006)]. By treating pairwise interactions as overlapping model systems, this integrated QM:QM approach essentially reduces the computation on a cluster into a 2-body:many-body problem. Consequently for a cluster composed of n fragments (or monomers), the high-level computations only need to be performed on at most n monomers and n(n − 1) / 2 pairs of fragments (or 'dimers'). The analytic gradients introduced here have been implemented in a stand-alone interface to the MPQC ab initio software package and subsequently applied to the geometry optimization of 15 different hydrogen bonded clusters of hydrogen fluoride, water and methanol. Structures were optimized with the RHF and MP2 methods as well as the MP2:RHF multicentred integrated QM:QM method. Geometrical parameters and energetics obtained with the MP2:RHF procedure are nearly identical to those from conventional MP2 computations despite the fact that the RHF results differ substantially from the MP2 target values. For example, the MP2:RHF dissociation energies are within 0.51kJmol− 1 (or 0.4%) of the MP2 results on average, while RHF always underestimates the MP2 dissociation energy by at least 12.86kJmol− 1 (or 20.4%). Similarly, the RHF hydrogen bond lengths are typically about 0.14Å too long while the MP2:RHF hydrogen bond lengths deviate from the MP2 values by only 0.0028Å on average.