ALBERT

Notes: Protonation/deprotonation reactions are represented by H++B(arrow-right-and-left)HB+. The ionization potential of H (13.6 eV) is higher than that of B for organic and most inorganic molecules (it is 10.166 eV for NH3), and the separated pair H+B+ will be lower in energy than the closed-shell pair H++B. The reaction path involves, therefore, an avoided crossing, and its theoretical study requires multideterminant methods. The reaction with B=NH3 (or R1R2R3N) is of interest in several fields, and its study is described here. The multireference coupled-cluster method (MR-CCM) and multireference double-excitation configuration interaction (MRD-CI) were used. At each (H3N---H)+ separation, from 1 to 11 bohr, the ground state MRD-CI energy was optimized with respect to the angle θ between the NH bond in the NH3 group and the C3 axis; MR-CCM and MRD-CI calculations were performed for the two lowest 1A1 states and the lowest 3A1. Two different reference determinants had to be used for the MR-CCM calculations at different regions, but this created no difficulties and the transition was smooth. Close agreement (a few mhartree) was obtained between MRD-CI and MR-CCM results. The avoided crossing, near R(H3N---H)+=4 bohr, is manifested by the rapid change in the CI and coupled-cluster method (CCM) mixing coefficients and by the transition of the NH3 group from pyramidal at small R to planar at large R. The lowest 1A1 state dissociates adiabatically to NH+3(2A1)+H(2S), whereas the single determinant self-consistent field (SCF) function dissociates to NH3(1A1)+H+.

Notes: The nature of interactions in the molecular crystal of urea is analyzed in terms of the interaction energy decomposition. The influence of the electron correlation effects was estimated on the basis of the calculated second order Møller–Plesset corrections and their analysis. In the crystal, the urea molecules form infinite ribbons which reveal strong cooperative effects. The hydrogen-bonded interactions of the orthogonal ribbons do stabilize the crystal, whereas interactions between parallel tapes are repulsive. The stability of the crystal structure is determined by a subtle balance between these two types of interactions. Although, the electron correlation effects are stabilizing, their contribution is rather small in comparison with the total interaction energy. © 2002 American Institute of Physics.

Notes: The CH3+Arn complexes are investigated for n up to 8. The molecular structures, dissociation energies, and vibrational frequencies of those complexes are studied, and the nature of the interactions is discussed. All possible structural isomers were considered. The formation of clusters is based on the consecutive filling of four distinct shells. The theoretically determined consecutive dissociation energies are in excellent agreement with experimental data derived from infrared photodissociation spectra and pulsed electron beam high-pressure mass spectrometry. The influence of an argon environment on the electron affinity of the CH3+ ion is also discussed. © 2001 American Institute of Physics.

Notes: The potential energy curves were studied for the proton transfer in the electronic ground and excited states for the model systems H3O2−, H4O2, and H5O2+. The complete active space self-consistent-field calculations were performed for the ground state optimized structures. The potential energy curves for the proton transfer in the excited states undergo a dramatic change due to the different electronic density distribution as an effect of electronic excitations. In all cases of the studied excited states, the electron population on the transferred proton is higher compared to that in the ground state. The total charge of the system greatly influences the potential curves. Energy separation between ground and excited states is decreased due to the negative charge of the system and is increased when the studied species are positively charged. The vertical excitations of the complex are similar to those in the monomers, but the proton relaxation leads to significant energetical (energy barriers) and structural (H+ position) changes. © 2001 American Institute of Physics.

Notes: The results of a theoretical study of the I−H2O anion and its neutral precursors are presented. The hydrogen-bonded structures were predicted for both the ionic and neutral complexes. The energetically preferred isomer for IH2O however is a species with the direct O–I bond. The relation between the potential energy surfaces for ionic and neutral moieties is evaluated based on their electron affinity properties. Thermodynamic and spectroscopic (IR) properties of complexes are discussed. The interaction energy decomposition is applied to explore the differences between the nature of bonding within the studied complexes. © 2001 American Institute of Physics.

Notes: The theoretical study of anionic and neutral halogen–water complexes is presented. The detachment of an electron from an anion leads to drastic changes in the structure and thermodynamic properties. Two possible neutral isomers separated by transition state were located. It is suggested that different neutral species are observed in photoelectron and mass spectrometry experiments. © 2001 American Institute of Physics.

Notes: Recently we extended our strategy for MRD-CI (multireference double excitation-configuration interaction) calculations, based on localized/local orbitals and an “effective” CI Hamiltonian, for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystalline or other solid environment.Our technique begins with an explicit quantum chemical SCF calculation for a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized, and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general in that the space treated explicitly, as well as the surrounding space, may contain voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc.Dimethylnitramine is the smallest prototype of the energetic R2N - NO2 nitramines, such as the 6-member ring RDX or the 8-member ring HMX. Decomposition of energetic compounds is initiated in the solid by a breaking of the target bond. Thus, it is crucial to know the difference in energy between breaking a bond in an isolated energetic molecule versus in the molecule in a solid. In the present study, we have carried out MRD-CI calculations for the Me2N - NO2 dissociation of dimethylnitramine in a dimethylnitramine crystal. The cases we investigated were one dimethylnitramine molecule (surrounded by 53 and 685 neighboring dimethylnitramine molecules represented by multipoles), three dimethylnitramine molecules, and three dimethylnitramine molecules (surrounded by 683 neighbors). All multipoles were cumulative atomic multipoles up through quadrupoles. The MRD-CI calculations on dimethylnitramine required large numbers of reference configurations from which were allowed all single and double excitations.

Notes: Within the presented LCAS MS (linear combination of atomic spinors-molecular spinors) SCF formalism both large and small components of the spinor radial parts have been expanded within the Gaussian basis set. The respective expressions for matrix elements as well as for one- and two-electron integrals are given.

Notes: Recently we derived, implemented, tested, and used successfully a new computational strategy for ab-initio MRD-CI (multireference double excitation - configuration interaction) calculations for molecular decompositions of large molecules and intermolecular reactions of large systems. We carry out the ab-initio SCF for the entire system, then transform the canonical delocalized molecular orbitals to localized orbitals and include explicitly in the MRD-CI only the localized occupied and virtual orbitals in the region of interest, folding the remainder of the occupied localized orbitals into an “effective” CI Hamiltonian. The advantage is that the transformations from integrals over atomic orbitals to integrals over molecular orbitals (the computer time-, computer core-, and external storage- consuming part of the CI calculations) only have to be carried out for the localized orbitals included explicitly in the MRD-CI calculations. The challenge arose to extend our MRD-CI technique based on localized/local orbitals and “effective” CI Hamiltonian to the breaking of a chemical bond in a molecule in a crystal (or other solid environment). This past year we have derived, implemented, and used successfully a procedure for doing this. Our technique involves solving a quantum chemical ab-initio SCF explicitly for a system of a reference molecule surrounded by a number of other molecules in the multipole environment of yet more further out surrounding molecules. The resulting canonical molecular orbitals are then localized and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general. The space treated explicitly quantum chemically and the surrounding space can have defects, deformations, dislocations, impurities, dopants, edges, and surfaces, boundaries, etc. We have applied this procedure successfully to the H3C - NO2 bond dissociation of nitromethane with extensive testing of the number of molecules that have to be included explicitly in the SCF and how many further out molecules have to be represented by multipoles. To check the goodness of the model cluster approximation for crystalline nitromethane, we carried out ab-initio crystal orbital (XTLORB) calculations using our POLY-CRYST program. The difference in the XTLORB total energies between the 4 nitromethane molecules/unit cell and the 3 nitromethane molecules/unit cell (Table VIII), ER = E4 - E3 = -48.0609079 a.u., corresponds very closely to the reduced energy per nitromethane molecule, ER = (-;48.0605)9 a.u., calculated from explicit SCF calculations on the model nitromethane cluster in the multipole field of farther out nitromethane molecules for the model cluster. Thus, the multipole approximation for describing the effect of further out molecules on the SCF cluster energies is quite good.