Debrecen, DFT15 Poster: Localized polycentric orbital basis set for Quantum Monte Carlo calculations derived from the decomposition of Kohn-Sham optimized orbitals

The Hohnberg-Kohn theorem establishes a one-to-one correspondence between the density and the external confining potential for a system of interacting particles. The consequence is that the ground state many body wave function depends only on the one particle density and the corresponding energy is a functional of the density itself. In the Kohn-Sham (KS) approach, such a density is reconstructed from the orbitals of an independent particle model. These building blocks, namely the KS orbitals, thus bring information of the real system. In this work, we present a simple decomposition scheme that, when applied on the KS optimized orbitals, is able to provide a reduced basis set, made of localized polycentric orbitals, specifically designed for Quantum Monte Carlo (QMC) calculations within the so called J-LGVBn theory}. The aforementioned decomposition follows a standard KS-DFT calculation and is based on atomic connectivity and shell structure. The new orbitals are used to construct a compact correlated wave function of the Slater-Jastrow form which is optimized at the Variational Monte Carlo level and then used as the trial wave function for a final Diffusion Monte Carlo accurate energy calculation. We are able, in this way, to capture the basic information on the real system brought by the KS orbitals and use it for the calculation of the ground state energy within a strictly variational method. Here, we show test calculations performed on some small selected systems to assess the validity of the proposed approach in a molecular fragmentation, in the calculation of a barrier height of a chemical reaction and in the determination of intermolecular potentials. The final Diffusion Monte Carlo energies are in very good agreement with the best literature data within chemical accuracy.