Global solutions of restricted open-shell Hartree-Fock theory from semidefinite programming with applications to strongly correlated quantum systems

TL;DR

This paper introduces a semidefinite programming approach to find global solutions of restricted open-shell Hartree-Fock theory, providing bounds and certificates of optimality, especially useful for strongly correlated quantum systems.

Contribution

It develops a novel SDP-based method for globally solving open-shell Hartree-Fock problems, extending previous closed-shell techniques with rigorous bounds and optimality guarantees.

Findings

Provides upper and lower bounds on Hartree-Fock energy

Guarantees global optimality when bounds coincide

Successfully applied to strongly correlated molecules

Abstract

We present a density matrix approach for computing global solutions of restricted open-shell Hartree-Fock theory, based on semidefinite programming (SDP), that gives upper and lower bounds on the Hartree-Fock energy of quantum systems. While wave function approaches to Hartree-Fock theory yield an upper bound to the Hartree-Fock energy, we derive a semidefinite relaxation of Hartree-Fock theory that yields a rigorous lower bound on the Hartree-Fock energy. We also develop an upper-bound algorithm in which Hartree-Fock theory is cast as a SDP with a nonconvex constraint on the rank of the matrix variable. Equality of the upper- and lower-bound energies guarantees that the computed solution is the globally optimal solution of Hartree-Fock theory. The work extends a previously presented method for closed-shell systems [S. Veeraraghavan and D. A. Mazziotti, Phys. Rev. A89, 010502R (2014)]. For strongly correlated systems the SDP approach provides an alternative to the locally optimized Hartree-Fock energies and densities with a certificate of global optimality. Applications are made to the potential energy curves of C2, CN, Cr2, and NO2.