Host: Professor Patrick Charbonneau
Abstract: Recent benchmark calculations for supramolecular complexes with 4-206 atoms have shown that relative errors in binding energies from second-order Møller-Plesset (MP2) many-body perturbation theory (MBPT) increase systematically with with the system size at a rate of approximately 0.1 % per valence electron. To analyze these unexpected results, an
asymptotic adiabatic connection symmetry-adapted perturbation theory (AC-SAPT) is presented which uses monomers at full coupling. Using the fluctuation–dissipation theorem, we obtain a nonperturbative “screened second-order” expression for the dispersion energy in terms of monomer quantities which is exact for non-overlapping subsystems and free of induction terms; a first-order random phase approximation (RPA) like approximation to the Hartree, exchange, and correlation kernel recovers the macroscopic Lifshitz limit. Explicit expressions for the convergence radius of the AC-SAPT series are derived within RPA and MBPT and numerically evaluated. Whereas the AC-SAPT expansion is always convergent for nondegenerate monomers when RPA is used, it is found to spuriously diverge for second-order MBPT, except for the smallest and least polarizable monomers. I will argue that the divergence of MBPT for presumably “weak” dispersion interactions is caused by missing or incomplete “electrodynamic” screening of the Coulomb interaction due to induced particle-hole pairs between electrons in different monomers, leaving the effective interaction too strong for AC-SAPT to converge within MBPT even in moderately polarizable molecules with a few tens of atoms. Conclusions for electronic structure theory and computational practice will be discussed.