Optimizing single Slater determinant for Electronic Hamiltonian with Lagrange multipliers and Newton-Raphson methods as an alternative to ground state calculations via Hartree-Fock self consistent field

TL;DR

This paper presents an alternative approach to Hartree-Fock ground state calculations using Lagrange multipliers and Newton-Raphson methods to optimize the single Slater determinant energy, offering potentially better convergence.

Contribution

It introduces a novel optimization method for the Hartree-Fock energy expression that avoids traditional eigensolvers, using Lagrange multipliers and Newton-Raphson techniques.

Findings

Demonstrated for closed shell systems

Potential for improved convergence

Applicable to restricted and open shell cases

Abstract

Considering the emblematic Hartree-Fock (HF) energy expression with single Slater determinant and the ortho-normal molecular orbits (MO) in it, expressed as a linear combination (LC) of atomic orbits (LCAO) basis set functions, the HF energy expression is in fact a 4th order polynomial of the LCAO coefficients, which is relatively easy to handle. The energy optimization via the Variation Principle can be made with a Lagrange multiplier method to keep the ortho-normal property and the Newton-Raphson (NR) method to find the function minimum. It is an alternative to the widely applied HF self consistent field (HF-SCF) method which is based on unitary transformations and eigensolver during the SCF, and seems to have more convenient convergence property. This method is demonstrated for closed shell (even number of electrons and all MO are occupied with both, alpha and beta spin electrons) and restricted (all MOs have single individual spatial orbital), but the extension of the method to open shell and/or unrestricted cases is straightforward.