Is HAM/3 (Hydrogenic Atoms in Molecules, Version 3) a semiempirical version of DFT (Density Functional Theory) for ionization processes?

Abstract

We calculated valence-electron vertical ionization potentials (VIPs) of nine small molecules, plus uracil and C2F4, by several different methods: semiempirical HAM/3 and AMI methods, different nonempirical DFT models such as uDI(B88-P86)/cc-pVTZ and -ε(SAOP)/TZP, and ab initio Hartree-Fock (HF) /cc-pVTZ. HAM/3 reproduced numerical values more closely to those calculated by the nonempirical DFTs than to those obtained by HF method. Core-electron binding energies (CEBEs) of aniline, nitrobenzene and p-nitro aniline, were also calculated by HAM/3 and nonempirical ΔFT using DE method. A nonempirical DFT model, designated as ΔEKS (PW86-PW91)/TZP model, resulted accurate CEBEs (average absolute deviation of 0.14 eV) with high efficiency. Although absolute magnitude of HAM/3 CEBEs has error as much as 3 eV, the error in the chemical shifts ΔCEBE is much smaller at 0.55 eV. While the CEBE results do not lead to any definite answer to the question in the title, the trends in valence-electron VIPs indicate that HAM/3 does not approximate DFT with accurate exchange-correlation potentials, but seems to simulate approximate functionals such as B88-P86.