Many-body effects in the X¹σ⁺states of the hydrogen fluoride, lithium fluoride and boron fluoride molecules

Many-body effects in the X1σ+ states of the hydrogen fluoride, lithium fluoride and boron fluoride molecules are investigated. Using diagrammatic perturbation theory, two, three and four-body contributions to the electronic energy are calculated to third order within the algebraic approximation. Many-body effects are found to be very significant. Two zeroth-order hamiltonians are employed and the convergence of the resulting perturbative expansions are compared. The [2/1] Padé approximants to the energy are constructed. Upper bounds to the energy are determined by evaluating the Rayleigh quotient for the first-order perturbative wavefunction.