Revisiting the potential energy surface for [H3N ��� HCl]: An ab initio and density functional theory investigation

Abstract

Theoretical calculations of the potential energy surface (PES) for the [NH, + HCl] system are presented using several standard ab initio methods such as Hartree-Fock (HF), second-order Mdller-Plesset perturbation theory (MP2), coupled cluster (CC), complete active space self-consistent-field (CASSCF), density functional theory (DFT), and less traditional ab initio approaches such as Dirac-Fock four-components and the use of effective Hamiltonian techniques, such as the recently proposed K functional. All calculations predict a single minimum for the complex, corresponding to a hydrogenbonded structure, confirming early studies. The dynamical and nondynamical contributions to the correlation energy are discussed for different cuts of the PES, involving different N-Cl distances. The complex has also been characterized by performing a full geometry optimization within the HF and DFT schemes; with the latter we have performed also the vibrational analysis. The predicted 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 f 0.5 kcal/mol, thus revising the experimental value of -8.0 k 2.8 kcal/mol; for the minimum, the predicted N-H and * is done with approximate inclusion of solvent effects (Onsager reaction field), the minimum is shifted and it corresponds to the ion pair NH:. C1-structure, similar to Mulliken's outer complex. Since the first ab initio computation for the NH,Cl 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 have concluded our comments with the invitation to use four-components Fock-Dirac for molecules both with high and low Z atoms, rather than the traditional Hartree-Fock and related methods. In 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 developments in computer hardware and software. In addition, the use of effective Hamiltonians, like the recently proposed " K functional," seems to deserve attention, because of their computational simplicity and physical reliability in predicting correlation corrections. 0 1996 John Wiley & Sons, Inc.

Ion-orbital coupling in Car-Parrinello calculations of hydrogen-bond vibrational dynamics: Case study with the NH3–HCl dimer

“…As expected the DFT calculations from previous work, in Ref. 15 for instance, and in our work here overbind the dimer. However, it is interesting to note that the second-order Møller-Plesset (MP2) calculations by Bevitt et al 19 find a binding energy of 0.41 eV that is larger than the value we find from the BLYP functional.…”

“…We calculated a value of 839 cm −1 for this mode. Thus, the frequency of the hydrogen-bond degrees of freedom calculated in this work are in reasonable agreement with the experimental data, given the known weaknesses of density functional theory (DFT) methods in describing hydrogen bonds, 3,15 and given that the proton-transfer arguments in Refs. 14 and 20 imply a higher H-Cl stretch frequency in the gasphase.…”

“…32,33 Our BLYP N-Cl distance 3.015 of is in modest agreement with the experimental data, and the PBE bond distance of 2.978 is even shorter. In this connection, it is interesting to note that the earlier DFT calculation by Corongiu et al, 15 using the Perdew-Wang functional 34 predict a N-Cl distance of 3.106 A which is in better agreement with the experimental data although the corresponding binding energy is considerably higher at 0.564 eV. The experimental gas-phase vibrational spectrum for the NH 3 -HCl dimer is not available because of its rather low-vapor pressure.…”

“…at a temperature of 25 K. As we have shown recently, 4 the frequency of a vibrational mode is linearly dependent upon the orbital mass, at least, for a harmonic normal mode. In Figure 1(a), we plot the vibrational spectra at 25 and 300 a.u.. By examining the projected vibrational density of states and in comparison with previous work, [13][14][15] we identify each peak in the spectra as indicated for the 25 a.u. spectrum.…”

“…We select the NH 3 -HCl dimer as the system to investigate because there is a considerable amount of previous work on this and closely related systems, and the vibrational frequencies have been investigated extensively, both experimentally [5][6][7][8][9][10][11][12][13][14] and theoretically. [14][15][16][17][18][19][20] In particular the influence of the anharmonic effects upon the vibrational frequency has been intensively studied, although the magnitude of the vibrational frequency shift due to anharmonic effects is still not clearly established, at least for the gas-phase dimer. This has been previously addressed by performing quantum dynamical calculations for one or two nuclear degrees of freedom propagating on a potential energy surface constructed using first-principles calculations.…”