title: An excited-state approach within full configuration interaction quantum Monte Carlo.
abstract: We present a new approach to calculate excited states with the full configuration interaction quantum Monte Carlo (FCIQMC) method. The approach uses a Gram-Schmidt procedure, instantaneously applied to the stochastically evolving distributions of walkers, to orthogonalize higher energy states against lower energy ones. It can thus be used to study several of the lowest-energy states of a system within the same symmetry. This additional step is particularly simple and computationally inexpensive, requiring only a small change to the underlying FCIQMC algorithm. No trial wave functions or partitioning of the space is needed. The approach should allow excited states to be studied for systems similar to those accessible to the ground-state method due to a comparable computational cost. As a first application, we consider the carbon dimer in basis sets up to quadruple-zeta quality and compare to existing results where available.
authors.affiliation: University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: N S
authors.initials: NS
authors.lastname: Blunt
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.
authors.forename: Simon D
authors.initials: SD
authors.lastname: Smart
authors.affiliation: Department of Physics, King's College London, Strand, London WC2R 2LS, United Kingdom.
authors.forename: George H
authors.initials: GH
authors.lastname: Booth
authors.affiliation: University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Ali
authors.initials: A
authors.lastname: Alavi
mesh_terms: D000465:Algorithms; D009010:Monte Carlo Method; D011789:Quantum Theory