Excited States, Dynamic Correlation Functions and Spectral Properties from Full Configuration Interaction Quantum Monte Carlo

TL;DR

This paper introduces a novel method within the Full Configuration Interaction Quantum Monte Carlo framework for accurately computing excited states, correlation functions, and spectral properties without approximations or complex orthogonalization.

Contribution

The method enables stable sampling of excited states and direct calculation of spectral functions, avoiding the need for trial wavefunctions or analytic continuation.

Findings

Successfully applied to neon atom and beryllium dimer

Allows direct computation of spectral functions

No need for explicit orthogonalization or trial wavefunctions

Abstract

In this communication, we propose a method for obtaining isolated excited states within the Full Configuration Interaction Quantum Monte Carlo framework. This method allows for stable sampling with respect to collapse to lower energy states and requires no uncontrolled approximations. In contrast with most previous methods to extract excited state information from Quantum Monte Carlo methods, this results from a modification to the underlying propagator, and does not require explicit orthogonalization, analytic continuation, transient estimators or restriction of the Hilbert space via a trial wavefunction. Furthermore, we show that the propagator can directly yield frequency-domain correlation functions and spectral functions such as the density of states which are difficult to obtain within a traditional Quantum Monte Carlo framework. We demonstrate this approach with pilot applications to the neon atom and beryllium dimer.