 
Summary: Electron correlation from path resummations: the doubleexcitation star
Alex J. W. Thom,
George H. Booth, and Ali Alavi
University of Cambridge, Chemistry Department, Lensfield Road, Cambridge CB2 1EW, U. K.
(Dated: November 12, 2007)
Resummation over a selected subset of paths allows the approximate evaluation of a Nelectron
pathintegral. In particular, we show that the double excitation star graph, consisting of all doubly
excited determinants attached to the reference HartreeFock determinant, has an energy which is
easily calculated in O
^
N4
~
time after integral precomputation, and produces binding curves of a
similar quality to CCSD theory for a range of systems including the N2 molecule, the hydrogen
bonded water dimer, and dispersion dominated Ne2 and Ar2 dimers.
I. INTRODUCTION
The calculation of the electronic correlation energy of
a molecule is fundamental for an accurate description
of inter and intramolecular interactions. The usual
wavefunctionbased approaches are based on manybody
