REVISITING THE POTENTIAL-ENERGY SURFACE FOR [H3N-CENTER-DOT-CENTER-DOT-CENTER-DOT-HCL] - AN AB-INITIO AND DENSITY-FUNCTIONAL THEORY INVESTIGATION

Theoretical calculations of the potential energy surface (PES) for the [NH3 + HCl] system are presented using several standard ab initio met hods such as Hartree-Fock (HF), second-order Moller-Plesset perturbati on theory (MP2), coupled cluster (CC), complete active space self-cons istent-field (CASSCF), density functional theory (DFT), and less tradi tional ab initio approaches such as Dirac-Fock four-components and the use of effective Hamiltonian techniques, such as the recently propose d K functional. All calculations predict a single minimum for the comp lex, corresponding to a hydrogen-bonded structure, confirming early st udies. The dynamical and nondynamical contributions to the correlation energy are discussed for different cuts of the PES, involving differe nt N-Cl distances. The complex has also been characterized by performi ng a full geometry optimization within the HF and DFT schemes; with th e latter we have performed also the vibrational analysis. The predicte d binding energies and infrared (IR) spectrum are compared with other theoretical and experimental results. For the gas phase, we propose a binding energy of -5.3 +/- 0.5 kcal/mol, thus revising the experimenta l value of -8.0 +/- 2.8 kcal/mol; for the minimum, the predicted N-H a nd H-Cl distances are 5.91 +/- 0.05 and 2.46 +/- 0.05 a.u., respective ly. When the computation is done with approximate inclusion of solvent effects (Onsager reaction field), the minimum is shifted and it corre sponds to the ion pair NH4+. Cl- structure, similar to Mulliken's oute r complex. Since the first ab initio computation for the NH4Cl complex is the pioneer work in 1967 by E. Clementi, the present work provides us with an opportunity to comment on some aspects of the evolution in computational chemistry, particularly for energy determinations. We h ave concluded our comments with the invitation to use four-components Fock-Dirac for molecules both with high and low Z atoms, rather than t he traditional Hartree-Fock and related methods;ln other words, we are of the opinion that the time is ready in quantum chemistry to switch from the Schrodinger to the Dirac representation, due to new developme nts in computer hardware and software. In addition, the use of effecti ve Hamiltonians, like the recently proposed ''K functional,'' seems to deserve attention, because of their computational simplicity and phys ical reliability in predicting correlation corrections. (C) 1996 John Wiley & Sons, Inc.