QMeCha: quantum Monte Carlo package for fermions in embedding environments

TL;DR

QMeCha is an open-source quantum Monte Carlo package designed to study complex many-body interactions involving fermions, nuclei, and environmental effects with advanced wavefunctions and efficient algorithms.

Contribution

It introduces a modular, expandable QMC code capable of modeling diverse quantum particles and environments with novel wavefunctions and optimization techniques.

Findings

Supports various wavefunctions including Pfaffian and Jastrow factors.

Models complex environments with classical charges and quantum oscillators.

Provides efficient variational and diffusion Monte Carlo protocols.

Abstract

We present the first open access version of the QMeCha (Quantum MeCha) code, a quantum Monte Carlo (QMC) package developed to study many-body interactions between different types of quantum particles, with a modular and easy-to-expand structure. The present code has been built to solve the Hamiltonian of a system that can include nuclei and fermions of different mass and charge, e.g. electrons and positrons, embedded in an environment of classical charges and quantum Drude oscillators. To approximate the ground state of this many-particle operator, the code features different wavefunctions. For the fermionic particles, beyond the traditional Slater determinant, QMeCha also includes Geminal functions such as the Pfaffian, and presents different types of explicit correlation terms in the Jastrow factors. The classical point charges and quantum Drude oscillators, described through different variational ans\"atze, are used to model a molecular environment capable of explicitly describing dispersion, polarization, and electrostatic effects experienced by the nuclear and fermionic subsystem. To integrate these wavefunctions, efficient variational Monte Carlo and diffusion Monte Carlo protocols have been developed, together with a robust wavefunction optimization procedure that features correlated sampling.