Relativistic Quantum Chemistry workflow in PySCF and Dice

PySCF (available at https://github.com/pyscf/pyscf) is an electronic structure package which has the capability to treat 4-component, 2-component as well as scalar relativistic Hamiltonians. With a recently implemented X2CAMF code (available at https://github.com/Warlocat/x2camf.) and its interface to PySCF, PySCF has obtained the ability to evaluate the spin-orbit coupling effects with both low cost and high accuracy. Various standard quantum chemistry methods including MP2, CCSD, EOMCC and TDDFT have been extended to treat the 2c Hamiltonian within PySCF to treat spin-orbit coupling.

Dice (available at https://github.com/sanshar/Dice) is an electronic structure package with an emphasis on stochastic electronic structure methods. Semistochastic Heat-bath Configuration Interaction (SHCI), stochastic variance of N-electron Valence state Perturbation Theory (NEVPT), various quantum Monte Carlo (QMC) methods including Variational Monte Carlo (VMC), Auxiliary-field Quantum Monte Carlo (AFQMC), Green's Function Monte Carlo (GFMF) and Full-Configuration Interaction Quantum Monte Carlo (FCIQMC) are all implemented in the package and the entire workflow is closely related with PySCF. Among these methods, SHCI and AFQMC have been extended to treat relativistic Hamiltonians. One can use SHCI handle over 100 spinors which is close to the relativistic implementation of MRCI at a near FCI quatlity. With AFQMC, correlating hundreds of spinors has also become possible. With selected CI wave function as a trial, the systematic bias in phaseless AFQMC is further reduced and can produce results at near FCI quality with an error at sub-millihartree scale.

PySCF in conjunction with Dice have provided a new toolset to the relativistic community to study more challenging systems with both standard methods and these emerging stochastic methods.