Many-body Study of Core-valence Partitioning and Correlation in Systems with Large-Z Element.

摘要

Quantum Monte Carlo (QMC) is one of the most promising many-body electronic structure approaches in studying strong correlated systems of electrons. We have applied QMC in the calculation of transition metal oxide and large systems containing heavy elements such as Thorium and Pb. Relativistic effect becomes non-neglible when heavy elements are involved. However, a direct calculation of relativistic effect in QMC is not practicable because it requires a 4-component framework and all of the electrons need to be considered. Scalar-relativistic effective core potential(ECP) and 2-component relativistic ECP(RECP) are introudced to quantum chemistry to mimic all-electron calculation including relativistic effects. We study the accuracy of the ECP of two different sizes of cores by Hartree-Fock, density functional theory(DFT) and QMC methods using MnO molecule as a test system.We show that the discrepancies between all-electron and ECP calulation of transition metal oxide is actually caused by the problem of non-linear exchange-correlation functionals in DFT, instead of the inaccuracy of the ECP. High accuracy diffusion Monte Carlo calculation of the MnO molecule confirms that the Ne-core and He-core ECPs are of comparable quality and therefore enable to reproduce energy differences within 0.1 eV or better accuracy margin. In addition, we have corroborated previous results on nodal surfaces which are more most accurate when using trial functions based on orbitals from hybrid functionals. We make a further modification on the ordinary QMC which extends the applicability to inherently complex wavefunctions. The complex state can result from a presence of magnetic field, boundary conditions or due to of spin-orbit interactions. The spin-orbit interactions is particularly interesting since that requires the spin to be dynamic unlike the spin-free mechanism in ordinary QMC which was restricted to a static label. We implement an inovative spinsampling technique and fixed-phase approximation for diffusion Monte Carlo(DMC). With the help of RECP, we calculate the excitation energy of Pb atom and binding properties of Pb molecules. The excellent agreement with experiment results shows our new spin-orbit QMC is very promising and capable to reproduce spin-orbit interaction. The study of thorium halides is completed in collaboration with Shi Guo and Shuming Hu. We investigate bond dissociation energies(BDE) of ThXn X=Cl,Br. Comparison of experiment results and theoretical calculation including DFT and QMC shows better agreement when using DMC on ThCln. However, an abnormal experimental BDE curve of ThBrn is not predicted by our calculations which indicates that additional work including both theoretical and experimental studies are very necessary to understand the unusal phenomenon of ThBrn.