Calculation of positron binding energies of amino acids with the any-particle molecular-orbital approach

We report positron binding energies (PBEs) for the 20 standard amino acids in the global minimum, hydrogen-bonded, and zwitterionic forms. The calculations are performed at the any-particle molecular-orbital (APMO) Hartree-Fock (HF), Koopmans' theorem (KT), second-order Möller-Plesset (MP2), and second-order propagator (P2) levels of theory. Our study reveals that the APMO KT and APMO P2 methods generally provide higher PBEs than the APMO HF and APMO MP2 methods, respectively, with only a fraction of the computational costs of the latter. We also discuss the impact of the choice of the positronic center on the PBEs and propose a simple and inexpensive procedure, based on the condensed Fukui functions of the parent molecules, to select the most suitable expansion center. The results reported so far indicate that APMO KT and APMO P2 methods are convenient options for a qualitative or semiquantitative analysis of positron binding in medium to large polyatomic systems.

Figure 1

The HF 6-311++G($d$,$p$) optimized structures of glycine: global minimum (left), hydrogen bonded (middle), and zwitterion (right).

Figure 2

Electronic density (orange) and positronic density (blue) for the hydrogen-bonded (left) and zwitterionic (right) conformations of the glycine $e^{+}$ system. The contour value of 0.0001 was used for all densities.

Figure 3

Positron binding energies and molecular dipole moments $\mu$ at the Hartree-Fock level of theory. The results of Koyanagi et al. are in red and our results are in blue. Circles and rhombi depict hydrogen-bonded structure data and triangles global minimum structure data.

Figure 4

Positron binding energies against the molecular dipole moments $\mu$. The circles indicate the present calculations and the straight lines are linear regressions. The results obtained with the (a) APMO HF, (b) APMO KT, (c) APMO MP2, and (d) APMO P2 methods are depicted, respectively, for the (1) HB structures and (2) ZW structures.