P. C. Hariharan

Multireference coupled cluster and multireference configuration interaction studies of the potential surfaces for deprotonation of NH+4

Protonation/deprotonation reactions are represented by H++B⇄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...

Electrostatic Molecular Potential Contour Maps from Ab-initio Calculations. 1. Biologically Significant Molecules. 2. Mechanism of Cationic Polymerization

In the portions of the hypersurface of the interaction energy between two molecules where polarization may be considered inessential to the understanding of the physical phenomenon under investigation, the interaction hypersurface may be well-approximated by electrostatic interactions. This approximation must be carefully controlled since its validity is not always well justified. In certain cases there may be a difference in exchange repulsion in a series of similar but not very closely related molecules approaching a substrate; however, such cases are the exception. The electrostatic potential arising from molecule A is completely defined at every point of the space if one knows the charge distribution (electronic and nuclear) of the molecule.1,2 When the electrostatic approximation is valid, electrostatic potential contour maps indicate vividly the potential field around a molecule as seen by an approaching reagent or a receptor site.

Procedure supplementing SCF interaction energies by dispersion term evaluated in dimer basis set within variation-perturbation approach

A variation-perturbation procedure for the evaluation of dispersion interaction, originally proposed by Jeziorski and van Hemert, has been reformulated to include basis set extension effects on an equal footing with the SCF interaction energy, corrected for basis set superposition error (BSSE). This approach has been tested for He2, (H2)2, (H2O)2, and (C2H4)2 complexes.

POLY-CRYST — A program for ab-initio crystal orbitals and polymer orbitals

Abstract Recently we have written a new program POLY-CRYST to calculate ab-initio orbitals for molecular crystals and polymers. This technique makes use of the translational symmetry in a crystal and the translational and/or translational rotational symmetry in a polymer. Into this new POLY-CRYST program we meshed, as options, all of our desirable optimal computational strategies for ab-initio calculations on large molecules. The results of preliminary test calculations are presented.

Ab initio MRD CI ground and excited state potential curves for addition of O to H2CCH2 and oxirane formation and decomposition

Abstract The addition of an oxygen atom across a CC bond is important in a wide variety of areas ranging from atmospheric chemistry and reactions involving energetic species to metabolic activation of polycyclic aromatic hydrocarbon carcinogens. The reverse process, decomposition of an epoxide ring is of interest in the same regard. We have carried out ab initio MRD CI calculations of the addition of O( 3 P g , 1 D g , 1 S g ) to the CC bond in C 2 H 4 to form C 2 H 4 O as a prototype system.

ENERGY TRANSFER IN Br +-Kr COLLISIONS

Abstract In the collision of Br + with Kr there is considerable transfer of translational to electronic energy and vice versa. This energy transfer is modelled as a Landau-Zener process at the points where the potential energy curves of the various electronic states of the halogen positive ion complex (KrBr + ) cross. Experimental transitions among the spin-orbit states of Br + (viz. 1 D 2 ⇌ 3 P 0,1 ) are observed but all attempts to produce 1 D 2 ⇌ 3 P 2 have failed. Relativistic Cl calculations of lowlying states of KrBr + have been carried out to explain the above experimental observations.

A new computational strategy for ab-initio MRD-CI calculations for breaking a chemical bond in a molecule in a crystal or other solid environment

Abstract 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-I 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, ER = E4 − E3 = − 48.0609079 a.u., corresponds very closely to the reduced energy per nitromethane molecule, ER = − 48.06057 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.

Ab-Initio Multireference Determinant Configuration Interaction (MRD-CI) and CASSCF Calculations on Energetic Compounds

The molecular decomposition pathways of >C — NO2 and >N — NO2 bonds are one of the key primary steps in initiation of explosives.

Crystal Structures of Energetic Compounds: Ab-Initio Potential Functions and Ab-Initio Crystal Orbitals

Detonation pressures and detonation velocities are governed by the crystal densities (gms./cc.) of explosives. Our approach to predicting optimal crystal-packing and crystal-structure parameters is based on ab-initio potential functions from nonempirical ab-initio calculations of smaller molecular aggregates (monomers, dimers, trimers, etc.). The total SCF interaction energies are partitioned into the different components, and then these components are fit individually to functional forms or when necessary recalculated or estimated explicitly for certain interaction components for each different unit cell dimension change. The CRYSTAL-JHU program, given the crystal symmetry, allows us to vary and optimize the crystal-structure parameters. The agreement of our calculated unit cell dimensions of nitromethane (CH3NO2) and of RDX with experiment was excellent, within 1 to 2.8%.