IMiCMO: a new integrated ab initio multicenter molecular orbitals method for molecular dynamics calculations in solvent cluster systems

A new computational scheme integrating ab initio multicenter molecular orbitals for determining forces of individual atoms in large cluster systems is presented. This method can be used to treat systems of many molecules, such as solvents by quantum mechanics. The geometry parameters obtained for three models of water clusters by the present method are compared with those obtained by the full ab initio MO method. The results agree very well. The scheme proposed in this article also intended for use in modeling cluster systems using parallel algorithms. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1107–1112, 2001     

Ab initio molecular orbital calculations on large lattice cluster models: Use of translational symmetry

Summary Translational symmetry has been shown to be useful in the calculation of electronic structures of large lattice models. The number of unique integrals has been derived for cases of different dimensionality. For the unique integrals zero screening and approximation methods are described. The method has been applied to arrays of hydrogen atoms and to a zincblende surface model. When the size of the system is increased the translationally unique integrals are shown to become either zero or they can be calculated by simple coulombic approximations.     

Ab initio molecular dynamics of protonated water clusters by integrated multicenter molecular-orbital method

To analyze large-scale cluster systems theoretically, we recently developed an "integrated multicenter molecular-orbital" (IMiC-MO) method. This method calculates the force of an entire system by dividing the system into small regions. We used the method to analyze the effect of cluster size and the process of hydrogen bond network (HBN) growth to form H(+)(H(2)O)(n) (n = 9, 17, and 33) clusters. Our simulations reveal that H(3)O(+) and water molecules in the first solvation shell function take an important role to grow the HBN. In addition, the number of hydrogen donors in each water molecule is strongly related to the shape of the HBN.     

Ab initio calculations on large molecules using molecular fragments. Initial studies on open shell systems

An ab initio procedure, designed for investigation of large molecules and based upon studies of molecular fragments, is extended to open shell systems using the unrestricted Hartree-Fock method. Investigated initially are the ethyl and vinyl radicals, and the ethylene triplet state.     

Ab initio centroid path integral molecular dynamics: application to vibrational dynamics of diatomic molecular systems

An ab initio centroid molecular dynamics (CMD) method is developed by combining the CMD method with the ab initio molecular orbital method. The ab initio CMD method is applied to vibrational dynamics of diatomic molecules, H2 and HF. For the H2 molecule, the temperature dependence of the peak frequency of the vibrational spectral density is investigated. The results are compared with those obtained by the ab initio classical molecular dynamics method and exact quantum mechanical treatment. It is shown that the vibrational frequency obtained from the ab initio CMD approaches the exact first excitation frequency as the temperature lowers. For the HF molecule, the position autocorrelation function is also analyzed in detail. The present CMD method is shown to well reproduce the exact quantum result for the information on the vibrational properties of the system.     

Ab initio quality one-electron properties of large molecules: development and testing of molecular tailoring approach

The development of a linear-scaling method, viz. "molecular tailoring approach" with an emphasis on accurate computation of one-electron properties of large molecules is reported. This method is based on fragmenting the reference macromolecule into a number of small, overlapping molecules of similar size. The density matrix (DM) of the parent molecule is synthesized from the individual fragment DMs, computed separately at the Hartree-Fock (HF) level, and is used for property evaluation. In effect, this method reduces the O(N(3)) scaling order within HF theory to an n.O(N'(3)) one, where n is the number of fragments and N', the average number of basis functions in the fragment molecules. An algorithm and a program in FORTRAN 90 have been developed for an automated fragmentation of large molecular systems. One-electron properties such as the molecular electrostatic potential, molecular electron density along with their topography, as well as the dipole moment are computed using this approach for medium and large test chemical systems of varying nature (tocopherol, a model polypeptide and a silicious zeolite). The results are compared qualitatively and quantitatively with the corresponding actual ones for some cases. This method is also extended to obtain MP2 level DMs and electronic properties of large systems and found to be equally successful.     

Ab initio calculations on large molecules using molecular fragments First order electronic properties for hydrocarbons

A number of first order electronic properties for the hydrocarbons C₂H₆, C₃H₈, C₂H₄, C₆H₆, and C₁₀H₈ are investigated. The wavefunctions employed here result from SCF calculations, using basis orbitals that have been optimized in molecular fragment studies. Comparisons are made with experimental values as well as with other calculated values, and the suitability of various molecular fragment bases is discussed.     

Ab initio SCF molecular dynamics: Exploring the potential energy surface of small silicon clusters

A few classical nuclear trajectories at finite temperature have been calculated from ab initio SCF energy gradients. They have been used as an alternative means to search for local minimum energy structures on the Born–Oppenheimer surfaces for some elemental silicon clusters. The approach is found to be beneficial in yielding different structures of silicon clusters. In other cases, the trajectories stay trapped in only one region of the phase space.     

Ab initio molecular orbital calculations for butadiene and its ions

Ab initio calculations are reported for cis and trans butadiene and some of their ions. The calculations are compared with semi-empirical results, and used to predict coupling constants.     

Ab initio quantum mechanics-based free energy perturbation method for calculating relative solvation free energies

A free energy perturbation (FEP) method was developed that uses ab initio quantum mechanics (QM) for treating the solute molecules and molecular mechanics (MM) for treating the surroundings. Like our earlier results using AM1 semi empirical QMs, the ab initio QM/MM-based FEP method was shown to accurately calculate relative solvation free energies for a diverse set of small molecules that differ significantly in structure, aromaticity, hydrogen bonding potential, and electron density. Accuracy was similar to or better than conventional FEP methods. The QM/MM-based methods eliminate the need for time-consuming development of MM force field parameters, which are frequently required for drug-like molecules containing structural motifs not adequately described by MM. Future automation of the method and parallelization of the code for Linux 128/256/512 clusters is expected to enhance the speed and increase its use for drug design and lead optimization.     

Ionic organoboranes. Part 9. Ab initio molecular orbital study of energy, structure, and frontier orbitals of the isomeric [7.7.10]ousenes

We have used HF/3-21G(*) geometry optimization to determine the relative energies, structures, symmetries, and nature of frontier orbitals for the seven isomeric [7.7.10^x,y]ousenes in which two cationic tropyliumyl (C₇H₆⁺-) rings are substituted on the cage of the B₁₀H₁₀²⁻ anion. The x,y=(2,7) isomer is the most stable. Energetically the remaining fall into two groups: (1,10), (2,4), and (1,6) which differ from (2,7) by less than 2 kcal mol⁻¹, and (2,6),(2,3), and (1,2) which differ by about 10 kcal mol⁻¹. The lower stability of the latter group is attributed to repulsive interactions between substituent rings rather than electronic effects. Three ousenes (1,2), (1,10), and (2,3) have lower symmetry than expected for a simple disubstituted B₁₀H₁₀²⁻ cage as a result of steric effects. Examination of frontier orbitals demonstrates that all seven species should show charge transfer excitations from cage to both rings similar to that previously experimentally observed for the [7.10²] hemiousenide ion.     

Ab initio simulations of liquid electrolytes for energy conversion and storage

Understanding physicochemical properties of liquid electrolytes is essential for predicting and optimizing device performance for a wide variety of emerging energy technologies, including photoelectrochemical water splitting, supercapacitors, and batteries. In this work, we review recent progress and open challenges in predicting structural, dynamical, and electronic properties of the liquids using first-principles approaches. We briefly summarize the basic concepts of first-principles molecular dynamics (FPMD), and we discuss how FPMD methods have enriched our understanding of a number of liquids, including aqueous solutions, organic electrolytes and ionic liquids. We also discuss technical challenges in extending FPMD simulations to the study of liquid electrolytes in more complex environments, including the interface between electrolytes and electrodes, which is a key component in many energy storage and conversion systems.     

Ab initio calculations and molecular dynamics simulation of H2 adsorption on CN3Be3+ cluster

Hydrogen adsorption properties of the CN₃Be₃⁺ cluster have been studied using density functional theory and MP2 method with a 6–31++G** basis set. Five hydrogen molecules get adsorbed on the CN₃Be₃⁺ cluster with a hydrogen storage capacity of 10.98 wt%. Adsorption of three H₂ molecules on one of the three Be atoms in a cluster is reported for the first time. It is due to the more positive charge on this Be atom than the remaining two. The average value for H₂ adsorption energy in CN₃Be₃⁺ (5H₂) complexes is 0.41 (0.43) eV/H₂ at MP2 (wB97XD) level, which fits well within the ideal range. Adsorption energy from electronic structure calculations plays an important role in retaining the number of H₂ molecules on a cluster during atom-centered density matrix propagation (ADMP) molecular dynamics (MD) simulations. According to ADMP-MD simulations, out of five H₂ adsorbed molecules on CN₃Be₃⁺, four and two H₂ molecules remain absorbed on CN₃Be₃⁺ cluster at 275 K and 350 K, respectively, during the simulation.

     

Ab initio potential energy surface and bound states for the Kr-OCS complex

The first ab initio potential energy surface of the Kr-OCS complex is developed using the coupled-cluster singles and doubles with noniterative inclusion of connected triples [CCSD(T)]. The mixed basis sets, aug-cc-pVTZ for the O, C, and S atom, and aug-cc-pVQZ-PP for the Kr atom, with an additional (3s3p2d1f) set of midbond functions are used. A potential model is represented by an analytical function whose parameters are fitted numerically to the single point energies computed at 228 configurations. The potential has a T-shaped global minimum and a local linear minimum. The global minimum occurs at R = 7.146 a(0), θ = 105.0° with energy of -270.73 cm(-1). Bound state energies up to J = 9 are calculated for three isotopomers (82)Kr-OCS, (84)Kr-OCS, and (86)Kr-OCS. Analysis of the vibrational wavefunctions and energies suggests the complex can exist in two isomeric forms: T-shaped and quasi-linear. The calculated transition frequencies and spectroscopic constants of the three isotopomers are in good agreement with the experimental values.     

Ab initio interpretation of Hund's rule for the methylene molecule: Variational optimization of its molecular geometries and energy component analysis

Hund's spin‐multiplicity rule for the ground state of the methylene molecule CH₂ is interpreted by Hartree‐Fock (HF) and multi‐reference configuration interaction (MRCI) methods. The stabilization of the triplet ground state ( ³B₁) relative to the second singlet excited state ( ¹B₁) is ascribed to the greater electron‐nucleus attraction energy that is gained at the cost of increasing the electron‐electron repulsion energy and with the aid of a reduction in the nucleus‐nucleus repulsion energy. The highest spin‐multiplicity in the ground state of CH₂ is accompanied by a set of three characteristic features, i.e., elongation of the internuclear distances, reduction in the bond angle, and contraction of the valence electron density distribution around the nuclei involving expansion of the core electron density distribution. The present calculations fulfill the virial theorem to an accuracy of ?V/T = 2.000 for both HF and MRCI. Accordingly, the molecular geometries are optimized for each of the two states. The inclusion of correlation by MRCI method reduces the energy splitting between the two states by about 14%. The energy splitting is analyzed by the correlational virial theorem 2T^c + V^c = 0 to make a clear interpretation of the correlation effect.© 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

A combined freeze-and-cut strategy for the description of large molecular systems using a localized orbitals approach

A technique to reduce the computational effort in calculating ab initio energies using a localized orbitals approach is presented. By exploiting freeze strategy at the self-consistent field (SCF) level and a cut of the unneeded atomic orbitals, it is possible to perform a localized complete active space (CAS-SCF) calculation on a reduced system. This will open the possibility to perform ab initio treatments on very large molecular systems, provided that the chemically important phenomena happen in a localized zone of the molecule. Two test cases are discussed, to illustrate the performance of the method: the cis-trans interconversion curves for the (7Z)-13 ammoniotridec-7-enoate, which demonstrates the ability of the method to reproduce the interactions between charged groups; and the cisoid-transoid energy barrier for the aldehydic group in the C13 polyenal molecule.     

Ab initio models for multiple-hydrogen exchange: Comparison of cyclic four- and six-center systems

High-level ab initio calculations {QCISD(T)/6-311 +G**//MP2(fu)/6-31 +G**, with corrections for higher polarization [evaluated at MP2/6-311 +G(3df,2p)] and ΔZPE//MP2(fu)/6-31 +G**, i.e., comparable to Gaussian-2 theory} indicate concerted mechanisms for double- and triple-hydrogen exchange reactions in HF and HCl dimers and trimers, in mixed dimers and trimers containing one NH₃, and in mixed dimers of HF, HCl, and NH₃ with formic acid. All these reactions proceed via cyclic four- or six-center transition structures, the latter being generally more favorable. Calculated activation barriers (ΔH^d? at 0 K, kcal/mol) are 42.3 for (HF)₂, 20.3 for (HF)₃, 41.2 for (HCl)₂, 25.6 for (HCl)₃, 36.0 for NH₃-HF, 10.6 for NH₃(HF)₂, 19.9 for NH₃-HCl, 2.3 for NH₃(HCl)₂, 9.7 for HCO₂H-HF, 7.0 for HCO₂H-HCl, and 11.3, for HCO₂H-NH₃. The barriers are lower for the more ionic systems and when more ion pair character is present. © John Wiley & Sons, Inc.     

Ab initio molecular orbital studies of the structure and potential energy surface of the LiAlF4 complex

Ab initio molecular orbital calculations have carried out on various structures of LiAlF₄ complex using minimal and extended basis sets. A C_2v structure with two fluorines in the bridge was found to be more stable than structures with one and three fluorines in the bridge. Migration of the Li⁺ in the complex is found to be relatively easy and the AlF^?₄ anion is found to be distorted from tetrahedral symmetry.