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Charge preservation is a necessary condition in population analysis. However, one such constraint is not enough to solve the arbitrariness involved in the population analysis such as Hirshfeld population. This arbitrariness results in too small Hirshfeld charges and poor reproducibility of molecular dipolar moments. In this article, the preservation of the molecular dipole moment is imposed upon the Hirshfeld population analysis as another constraint to improve the original Hirshfeld charges. In the scheme each atomic dipolar moment defined by the deformation density is expanded as contributions from all atoms in the molecule. The corresponding correction charges are then accumulated for each atom together with the original Hirshfeld charge as the predicted charge. All computed charges are generally larger than Hirshfeld charges, independent of basis set, and have very good electrostatic potential reproducibility and high correlation with the charges derived from the electrostatic potential fitting.

A few open-shell molecules are taken as examples in order to examine the performance of the open-shell perturbation theory for electron correlation (J Chem Theory Comput, 2009, 5: 931–936). The convergence of the perturbation series is shown to be stable for the doublet state of NH2 at both the equilibrium and stretched geometries. The equilibrium bond lengths, and harmonic and anharmonic vibrational frequencies are calculated for NO(X 2Π), OH(X 2Π), CH(X 2Π) and NH(X 2Σ−) with different second-order perturbation theories at the cc-pVDZ, cc-pVTZ and cc-pVQZ levels. The ground state energies of BeF(X 2Σ+), MgH(X 2Σ+) and the HCCl triplet state have also been computed with various perturbation theories and compared with configuration interaction with single and double excitations (CISD) and CISD + Davidson correction. The energy difference between the formaldehyde (H2CO+) and hydroxymethylene (HCOH+) radical cations has been computed. Our perturbation theory predicts correctly that H2CO+ is more stable than HCOH+. However, calculations using UMP2, CASPT2, the Z-averaged perturbation theory and restricted Moller-Plesset theory fail even to produce the correct sign of the energy difference.

Photoabsorption cross-sections of water dimer and trimer anions are calculated with an ab initio molecular orbital method. Because the electron detachment energy of these small water cluster anions is less than 0.3 eV, all of the photoabsorption spectra in the near-infrared and visible region are due to the bound-free transitions. The theoretical spectra of two isomers of dimer anion and three isomers of trimer anion are compared, which suggest the possibility of the identification of the isomer structure by the photoabsorption spectra. The convergence on the basis sets is also examined by adding diffuse functions.

Electronic spectra of ammoniated ammonium radicals, NH4(NH3)n, produced through the photolysis of ammonia clusters, are investigated with a laser photodepletion spectroscopy. Vibrationaly resolved bands beginning at energy of 9305 ± 15 cm-1 are successfully assigned to NH4NH3. These bands are ascribed to the 2 2A −1 2A transition derived from the 3p 2F2−3s 2A1 excitation of NH4. A second group of bands beginning at energy 10073 ± 15 cm-1 is assigned to the 12E−12A transition. Electronic spectra of a series of NH4(NH3)n (n = 1−8) are also recorded with low resolution in the energy region of 4500−16000 cm-1. A drastic decrease of the excitation energy from 15062 cm-1 for NH4 to 5800 cm-1 for NH4(NH3)4 is observed, while no appreciable spectral change is found for n ≥ 5. The large spectral change is ascribed to the spontaneous ionization of NH4 in ammonia clusters. Successive binding energies of NH4(NH3)n in the excited state are determined from the spectral band positions.

The geometrical structures of NH4 and OH3 were optimized at the MP2 level with the aug-cc-pvdz, aug-cc-pvtz, and aug-cc-pvqz basis set plus s-type diffuse functions. The adiabatic ionization potential, barrier height, and dissociation energy of NH4 and OH3 were calculated with the above basis set series and were extrapolated to the infinite basis set limit. The theoretical ionization potential of NH4 was in very good agreement with the experimental value. The N−H bond lengths of NH4 and NH4+ at the infinite basis set limit were obtained by parabolic interpolation around the equilibrium point. FH2 was optimized at the UHF, B3LYP, and MP2 levels. However, only a dissociated ground state was found. The potential energy curves for dissociation of the above molecules were calculated with MP2. The relative stabilities of these molecules and their isotopic counterparts are discussed. Theoretical hyperfine calculations were performed in the gas phase as well as in a neon matrix for NH4 and OH3 with a single-refer...

Ab initio calculations have been performed to interpret the photoelectron spectrum of gaseous cuprous chloride, Cu3Cl3. Density functional calculations revealed Cu3Cl3 to be a planar cyclic D3h molecule. Koopmans' theorem and two-hole/one-particle calculations with canonical Hartree-Fock orbitals were used to interpret the vertical ionization energies. These were compared with similar calculations using B3LYP Kohn-Sham orbitals. The results confirm the claim by Casida that Kohn-Sham orbitals mimic Dyson orbitals.

The structure of the CH2OH radical was optimized at the UHF, UB3LYP, and MP2 levels. The calculated ionization potential was very close to the experimental one. The transition dipole moments from the ground state to the first four lowest excited states were calculated with and without Cs symmetry using the multireference single and double configuration interaction method (MRSD). The first four lowest excited states were also optimized with the single excitation configuration interaction method (CIS). The adiabatic excitation energies and transition dipole moments for vertical emission were calculated at the same level. None of these calculations lead to the assignment of the first excited Rydberg p state as 2A‘ ‘(3pz) as suggested in a recent paper (Chem. Phys. Lett. 318, 393, 2000). Some results of multireference perturbation theory are also presented. The isotropic hyperfine parameters calculated with MRSD were in good agreement with the experimental values.

Electronic structure calculations on ions that use dielectric continuum theory to mimic solvent around the bare ionic solutes are often prone to make large errors in the hydration energies. It is found for cations that much of the error can be accounted for by a simple linear correlation with the maximum value found anywhere on the dielectric cavity surface of the solute potential or, even better, the outgoing normal electric field, thus mirroring analogous results previously obtained for anions. This correlation allows for significantly improved estimates of cation hydration energies while still retaining cavities of physically reasonable size in determination of the bulk dielectric contributions.