Pushing Configuration-Interaction to the Limit: Towards Massively Parallel MCSCF Calculations

TL;DR

This paper introduces a highly parallelized MCSCF implementation in NWChem, enabling large active space CI calculations previously considered infeasible, demonstrated on complex systems like chromium clusters and pentacene.

Contribution

The paper presents a novel massively parallel MCSCF approach using GAS in NWChem, allowing large active space CI calculations with unprecedented scale.

Findings

Performed CI calculations with 22 electrons in 22 orbitals.

Achieved a CI expansion of over one trillion SDs for chromium tetramer.

Enabled single iteration CI calculations with 24 electrons in 24 orbitals.

Abstract

A new large-scale parallel multiconfigurational self-consistent field (MCSCF) implementation in the open-source NWChem computational chemistry code is presented. The generalized active space (GAS) approach is used to partition large configuration interaction (CI) vectors and generate a sufficient number of batches that can be distributed to the available nodes. Massively parallel CI calculations with large active spaces can be treated. The performance of the new parallel MCSCF implementation is presented for the chromium trimer and for an active space of 20 electrons in 20 orbitals. Unprecedented CI calculations with an active space of 22 electrons in 22 orbitals for the pentacene systems were performed and a single CI iteration calculation with an active space of 24 electrons in 24 orbitals for the chromium tetramer was possible. The chromium tetramer corresponds to a CI expansion of one trillion SDs (914 058 513 424) and is largest conventional CI calculation attempted up to date.