Nonempirical wave functions for very large molecules. II. The PRDDO/M/FCP method

A variation of the frozen-core potential (FCP) method is developed and implemented within the modified version of the method of partial retention of diatomic differential overlap (PRDDO/M). The explicit treatment of core electrons is replaced with a potential based on the actual core-valence integrals rather than upon an arbitrary model potential. The core-valence orthogonality requirement is replaced by an energy shift operator. PRDDO/M/FCP calculations exhibit good agreement with ab initio calculations with the same basis set, while reducing the computational cost significantly. © 1997 John Wiley & Sons, Inc.     

ESR study and ab initio calculations of some methylbenzoates derivatives.

The preferred twisted conformation in orthomethylbenzoic acids is analyzed by means of an ab initio calculation. However, the esr data indicate a planar structure in the orthomethylbenzoates. The increase of the conjugation in the ester radical anions is also analyzed by OS-SCF ab initio calculations using an STO 3G basis set.     

Water-Chain intervention in the ketonization of vinyl alcohol. An ab initio study

The ketonization of vinyl alcohol in aqueous solution has been studied by means of ab initio calculations, using the STO -3G basis set. The intervention of a chain of two water molecules reduces dramatically the potential barrier. This result agrees with experimental facts.     

An ab initio study of intermolecular hydrogen bonding between small peptide fragments

Several small peptide fragments are investigated with ab initio (Hartree-Fock) calculations, using Gaussian basis sets. Complexation energies, net atomic charges, and optimum geometries are obtained. The geometries predicted by the STO -6G, and 6–31G* basis sets are quite similar, whereas the binding energies obtaiend by the 6–31G calculations are higher than those obtained with STO -6G and 6–31G* basis sets.     

Ab initio studies of arginine and deaminoarginine

L-Arginine and Deaminoarginine were studied via quantum chemical ab initio calculations using the STO -3G basis set for arginine and the STO -3G and 3–21G basis sets for deaminoarginine. It was found that the most stable conformations are the ones featuring the carboxyl group slightly twisted versus the rest of the molecule, which adopts an extended conformation. It was also found that the cyclic zwitterion is less stable than the neutral cyclic conformation and that there is a barrier to the proton transfer from the carboxyl's oxygen to one of guanidine moiety's nitrogens.     

Green's function calculation of through‐bond electronic coupling in donor–bridge–acceptor model systems II: Importance factors applied to atomic sites

Importance factors, associated with the Green's function formalism, are introduced. They are applied for the determination of the relative atomic site contribution to the electronic interaction propagation in a molecular system. The calculation is performed at the Hartree–Fock (self‐consistent) level, using ab initio STO‐3G, 4‐31G, and D95 basis sets. The results are compared with those obtained from the charge densities of the appropriate molecular orbitals at the ab initio STO‐3G level. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

某些含氧酸中原子净电荷与pK_a的关系

在量子化学从头算 (分别采用HF/STO 3G ,HF/STO 3G ,HF/ 3 2 1G和MP2 /STO 3G)水平下 ,对 11种含氧酸分子 (H2 O ,H2 O2 ,HNO2 ,HNO3 ,HClO ,HClO2 ,HClO3 ,HCO2 H ,CH3 CO2 H ,HBrO和HIO)进行几何构型全优化和电子结构计算 ,将获得的电子结构数据与 pKa 实验值进行逐步回归分析 ,结果发现含氧酸中羟基氢原子净电荷、含氧酸中与羟基氧相连非氢原子净电荷两项与 pKa 呈良好的二元线性关系 ,表明含氧酸中羟基氧所连原子的净电荷在决定含氧酸的酸性上起重要作用     

Accurate molecular electrostatic potentials based on modified PRDDO/M wave functions. I. Electrostatic potential derived atomic charges

A new approach for the calculation of electrostatic potential derived atomic charges is presented. Based on molecular orbital calculations in the PRDDO/M approximation, the new parametrized electrostatic potential (PESP) method is parametrized against ab initio MP2/6-31G** calculations. For a data set of 820 atoms in 145 molecules containing H, C, N. O, F, P, S, Cl, and Br (including hypervalent species), the PESP method achieves a mean absolute error of 0.037 e⁻ with a correlation coefficient of 0.990. Unlike other approximate approaches, no scaling factor is required to improve the agreement between PESP charges and the underlying ab initio results. PESP calculations are an order of magnitude faster than the simplest ab initio calculation (STO-3G) on large molecules while achieving a level of accuracy that rivals much more elaborate ab initio methods. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 955–969, 1997     

Molecular calculations with the MODPOT,VRDDO, and MODPOT/VRDDO procedures. IV. Boron hydrides and carboranes

Reference completely ab initio 6–3G and nonempirical 3G/MODPOT (ab initio effective core model potential) LCAO -MO -SCF calculations (using the same valence atomic orbital basis) were performed for a series of boron hydrides (B₄H₁₀, B₅H₉, B₅H₁₁, and B₆H₁₀) and a test 3G/MODPOT + VRDDO (variable retention of diatomic differential overlap for charge conserving integral prescreening) calculation were also performed for B₅H₉, B₆H₁₀, and B₁₀H₁₄. The agreement between the ab initio 6–3G and the corresponding 3G/MODPOT calculations was excellent for valence orbital energies, gross atomic populations, and dipole moments. The results also compared favorably to previous ab initio minimum STO basis results of Lipscomb and coworkers. The 3G/MODPOT + VRDDO calculations verified that for such spatially compact molecules (such as boron hydrides, which are fragments of polyhedra), the VRDDO procedure does not result in a noticeable savings in computer time for molecules of the size and shape of B₅H₉ and B₆H₁₀, in contrast to the savings previously realized for organic molecules of comparable atomic size. However, the agreement in calculational results between the 3G/MODPOT and the 3G/MODPOT +VRDDO results was still as extremely close as it had been for the organic molecules. 3G/MODPOT calculations were also carried out for B₈H₁₂, B₉H₁₅, B₁₀H₁₄, B₁₀H₁₄^?2, 1,2-C₂B₄H₆, and 1,6-C₂B₄H₆ and the results compared to the previous minimum STO basis results. For B₁₀H₁₄, the 3G/MODPOT +VRDDO method led to savings in computer time of 28% over the 3G/MODPOT method itself. The agreement of the 3G/MODPOT results with available experimental photoelectron spectral data for B₅H₉ and 1,6-C₂B₄H₆ was as good as that of the previous ab initio minimum STO basis calculations.     

The carbonyl group as homoconjugated electron donating substituent. Ab initio sto 3G mo calculations.

ab initio STO 3g calculations on 2,3-episulfonionorbornanes and 2-norbornyl cations substituted at C(5) or C(6) suggest that homoconjugated carbonyl group and related (-M, -I) functions can be remote electron-donors.     

Anab initio SCF molecular orbital study of acetylcholine

A standard ab initio (STO 3 G) study indicates, in distinction to theab initio molecular fragment results, that the preferred conformation of acetylcholin is gauche.     

紫杉醇的核磁共振谱及其分子几何构型的从头算研究

应用规范不变原子轨道GIAO法在HF/6 31G水平计算了MM3、PM3和HF/STO 3G三种最优分子构型下紫杉醇的δ 1H NMR和δ 13C NMR.对NMR化学位移计算值与实验值的对比和误差的统计和分析表明,采用HF/STO 3G优化的几何构型计算得到的化学位移误差最小，因此这一构型与实际分子最为接近.从理论构型得到4 OAc与3′ H的距离为0.266 3 nm，4 OAc与3′ Ph的距离为0.310 4 nm，4 OAc与2 OCOPh的距离为0.370 7nm，支持了Williams等关于紫杉醇分子内存在NOE (nuclear overhauser effect)效应及Velde等关于在极性溶剂中4 OAc与3′ Ph、2 OCOPh形成分子内疏水簇的观点.     

Theoretical studies on the conformations and properties of phenylazonaphthalenes

The conformations of phenylazo‐2‐naphthalene (I) and phenylazo‐1‐naphthalene (II) have been studied using ab initio methods and density functional theory. The rotational potential energy surfaces about the C N bonds were calculated for both the trans and the cis forms at the PM3 and HF/STO‐3G levels. The PM3 method was found to be incapable for locating the energy minima correctly. The geometries of rotamers obtained from the HF/STO‐3G surface were fully optimized at the HF/6‐31G* and B3LYP/6‐31G* levels. Single‐point MP2/6‐31G* calculations have also been carried out at the HF/6‐31G* geometries. The vibrational spectra, standard thermodynamic functions, heats of formation, and equilibrium molar fractions have been obtained. According to the calculated results, I is comparatively more stable than II. The trans forms have much lower energies than the cis forms, implying that I and II mainly exist in the trans planar forms. The energy gap (ΔE) between the frontier molecular orbitals (MOs) of the cis forms are about 0.7 eV higher than those of the trans ones, indicating that change in geometry from trans to cis form will cause a hypsochromic effect. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 25–33, 2000     

Simplified ab initio calculations on small molecules with an extended balanced approximate MO formalism

Based on a test of different known simplified ab initio methods an approximate LCAO MO formalism is proposed which gives reasonable results in a minimal STO basis in comparison with ab initio calculations.     

Systematic errors in the total energy of molecular wave functions calculated within the PRDDO approximations

We compare calculated total energies for 150 open-chain molecules using ab initio methodology and the PRDDO approximations. The bulk of the errors implicit in the PRDDO approximations are apparently of a one-center nature, i.e., they are due to the number and type of atoms in the molecule, and not the details of the molecular geometry. Atomic correction factors are developed which reduce the errors in the calculated total energy of PRDDO wave functions by a factor of eight relative to the ab initio reference calculations. PRDDO calculations on ring and cage compounds are shown to have additional systematic errors in the total energy.     

Calculations on the diphenylmethyl anion

Semiempirical and ab initio calculations for the diphenylmethyl anion and related species are reported. MINDO/3 calculations indicate a coplanar anion with an enlarged bond angle of ～139° to counteract the steric repulsions. MNDO calculations reveal an expanded bond angle with somewhat greater twist (21°). The ab initio calculations (STO -3G level) reveal an expanded central bond angle with an intermediate degree of phenyl twist. The results are compared with experimental data for this and related anions.     

Ab initio calculation of the electronic structure and the barrier to pyramidal inversion for H3S+

The electronic structure of H₃S⁺ is examined by ab initio MO calculations (STO 3G) and is compared with those of other XH₃ type molecules. The barrier to pyramidal inversion and the proton affinity of H₂S are calculated to be 34.8 and 225.05 kcal/mol respectively. Computations are made with respect to a model A₃S⁺ which is employed to discuss the barrier to pyramidal inversion of H₃S⁺, where A corresponds to an electromagnetive substituent or an electron donating one.     

Excited states and photochemistry of saturated molecules. ab initio calculations on methane

STO 4G calculations are carried out on methane in both tetrahedral and Jahn—Teller distorted C_2v and C_3v symmetries. Large increases in CH bond lengths are predicted in all states, and the ^IB₁ state in C_2v symmetry is predicted to dissociate to ^IB₁ methylene and ¹A₁ H₂ width no energy barrier. THe results are compared with earlier INDO calculations, and the latter are found to qualitatively consistent with the ab initio results.     

An MS-Xα study of the photoelectron spectra and bonding in the fluorosilanes

Calculations with the MS-Xα method, with and without overlap, STO 3G ab initio and CNDO/2, with different basis sets, were performed for the fluorosilane series. The valence ionization potentials evaluated with the transition state formalism are in essential agreement with the assignment previously proposed for the photoelectron spectra. Various inversions are predicted by the other methods. The electronic structure, orbital population and silicon d participation to the bond are discussed.     

Deorthogonalization of atomic originals in the CNDO approach

The procedure for deorthogonalization (D) of atomic orbitals in the semiempirical CNDO approach is reviewed. For comparative studies, CNDO/2, CNDO/2D, and STO -3G calculations of molecular dipole moments and Mulliken populations are carried out on 35 prototype molecules containing H, C, N, O, and F atoms. The calculated values are assessed on the basis of how well they agree with experimental trends, chemical bonding theories, and ab initio molecular orbital (MO) values. Results of analyses indicate that the CNDO/2D values for dipole moments are in reasonable agreement with experimental values, and those for net atomic charges and electron populations bear greater resemblance to the ab initio (STO -3G and 6-31G**) values than the original CNDO/2 values. These findings, together with those of previous investigators, demonstrate unequivocally the advantages of incorporating deorthogonalization into routine CNDO/2 or INDO calculations as a means to obtain reasonable estimates of charge distributions.