3-PHONON VERTEX IN DESCRIPTION OF GROUND STATE OF LIQUID HE

The second order energy generated by three-phonon components,ρkρlρ-k- ψ with kl|k+1| ≠ o, in the ground state wavefunction of liquid He4, is evaluated under the assumption that ψ is the optimum BDJ type trial function (in the sense that E = ∫ ΨHΨ attains a minimum value Eopt. within the BDJ function space). The three-particle polarization energy is also evaluated. Both energy corrections are large (10-20% of Eexp.) and strongly dependent on density. However, they are of opposite sign and, with crude estimates, cancel effectively so that the direct comparison of E (or better Eopt.) with Eexp. is meaningful. © 1969.