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The linear TiCH molecule is studied by ab initio quantum mechanical calculations using quantitative basis sets and highly correlated computational methods. Potential energy curves along the Ti2CH coordinate have been computed to obtain a better understanding of molecular formation in eight low-lying states of the molecule. Total energies, dissociation energies (with respect to Ti +CH), equilibrium distances, and dipole moments are reported. Simple valence bond Lewis diagrams are used to interpret the nature of the bonding in all of the states studied. [ABSTRACT FROM AUTHOR]

The correlation-consistent polarized valence basis sets (cc-pVXZ) for the atoms boron through neon have been extended to treat core and core-valence correlation effects. Basis functions were added to the existing cc-pVXZ sets to form correlation-consistent polarized core-valence sets (cc-pCVXZ) in the usual pattern: Double zeta added (1s1p), triple zeta added (2s2p1d), quadruple zeta added (3s3p2d1f), and quintuple zeta added (4s4p3d2f1g). The exponents of the core functions were determined by minimizing the difference between all-electron and valence-only correlation energies obtained from HF+1+2 calculations on the ground states of the atoms. With the cc-pCVXZ sets, core, core-valence, and valence correlation energies all converge exponentially toward apparent complete basis set (CBS) limits, as do the corresponding all-electron singles and doubles CI energies. Several test applications of the new sets are presented: The first two ionization potentials of boron, the 3P–5S separation in carbon, and the X 3B1–a 1A1 state separation in CH2. As expected, correlation effects involving the core electrons of the first row atoms, B–Ne, are small but must be included if high accuracy is required. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An accurate description of the electrical properties of atoms and molecules is critical for quantitative predictions of the nonlinear properties of molecules and of long-range atomic and molecular interactions between both neutral and charged species. We report a systematic study of the basis sets required to obtain accurate correlated values for the static dipole (α1), quadrupole (α2), and octopole (α3) polarizabilities and the hyperpolarizability (γ) of the rare gas atoms He, Ne, and Ar. Several methods of correlation treatment were examined, including various orders of Moller–Plesset perturbation theory (MP2, MP3, MP4), coupled-cluster theory with and without perturbative treatment of triple excitations [CCSD, CCSD(T)], and singles and doubles configuration interaction (CISD). All of the basis sets considered here were constructed by adding even-tempered sets of diffuse functions to the correlation consistent basis sets of Dunning and co-workers. With multiply-augmented sets we find that the electrical properties of the rare gas atoms converge smoothly to values that are in excellent agreement with the available experimental data and/or previously computed results. As a further test of the basis sets presented here, the dipole polarizabilities of the F- and Cl- anions and of the HCl and N2 molecules are also reported. [ABSTRACT FROM AUTHOR]

In the past, basis sets for use in correlated molecular calculations have largely been taken from single configuration calculations. Recently, Almlöf, Taylor, and co-workers have found that basis sets of natural orbitals derived from correlated atomic calculations (ANOs) provide an excellent description of molecular correlation effects. We report here a careful study of correlation effects in the oxygen atom, establishing that compact sets of primitive Gaussian functions effectively and efficiently describe correlation effects if the exponents of the functions are optimized in atomic correlated calculations, although the primitive (sp) functions for describing correlation effects can be taken from atomic Hartree–Fock calculations if the appropriate primitive set is used. Test calculations on oxygen-containing molecules indicate that these primitive basis sets describe molecular correlation effects as well as the ANO sets of Almlöf and Taylor. Guided by the calculations on oxygen, basis sets for use in correlated atomic and molecular calculations were developed for all of the first row atoms from boron through neon and for hydrogen. As in the oxygen atom calculations, it was found that the incremental energy lowerings due to the addition of correlating functions fall into distinct groups. This leads to the concept of correlation consistent basis sets, i.e., sets which include all functions in a given group as well as all functions in any higher groups. Correlation consistent sets are given for all of the atoms considered. The most accurate sets determined in this way, [5s4p3d2f1g], consistently yield 99% of the correlation energy obtained with the corresponding ANO sets, even though the latter contains 50% more primitive functions and twice as many primitive polarization functions. It is estimated that this set yields 94%–97% of the total (HF+1+2) correlation energy for the atoms neon through boron. [ABSTRACT FROM AUTHOR]

Using systematic sequences of correlation consistent Gaussian basis sets from double to sextuple zeta quality, the classical barrier height of the H+H2 exchange reaction has been calculated by multireference configuration interaction (MRCI) methods. The MRCI calculations for collinear H3 have also been calibrated against large basis set full CI (FCI) results, which demonstrate that the MRCI treatment leads to energies less than 1 μhartree (≤0.001 kcal/mol) above the FCI energies. The dependence of both the H2 and H3 total energies on the basis set is found to be very regular, and this behavior has been used to extrapolate to the complete basis set (CBS) limits. The resulting estimate of the H–H–H CBS limit yields a classical barrier height, relative to exact H+H2, of 9.60±0.02 kcal/mol; the best directly calculated value for the barrier is equal to 9.62 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo calculations. [ABSTRACT FROM AUTHOR]

Using correlation consistent basis sets from double through quintuple zeta quality, potential energy functions have been calculated for the electronic ground states of the first row homonuclear diatomic molecules B2, C2, N2, O2, and F2 using single and double excitation configuration interaction (HF+1+2, GVB+1+2, and CAS+1+2) wave functions. Spectroscopic constants have been calculated for each species and compared to experiment. The dependence of the calculated spectroscopic constants on systematic extensions of the one-particle basis set are, in general, found to be very regular. By fitting the directly calculated values with a simple exponential function, accurate estimates of the complete basis set (CBS) limit for Ee, De, and re have been obtained for each level of theory. The estimated CBS limits are compared to the available experimental results, and the intrinsic errors associated with each theoretical method are discussed. In addition, the accuracy of the internally contracted CAS+1+2 method is compared to conventional uncontracted calculations using large basis sets. For B2, a full CI calculation have been carried out for De with the correlation consistent double zeta basis set and is compared to the CAS+1+2 method using both a supermolecule and separated atom approach for the dissociated limit. [ABSTRACT FROM AUTHOR]

Abstract: The nature of the bonding in hypervalent molecules has long been a topic of discussion with a number of explanations being offered to rationalize the apparent violation of the octet rule. By examining the formation of the second row fluorides, XF n −1 +F→XF n (X=P, S, Cl), we found that a new type of bond is present in these compounds, the recoupled pair bond, which is distinct from other types of chemical bonds. A recoupled pair bond occurs when it is energetically favorable to uncouple an existing pair of electrons to form bonds, which is favorable for most of the valence s 2 and p 2 pairs in P, S, and Cl. The interplay between the formation of normal covalent bonds and recoupled pair bonds in the XF n species readily explains the structures of these species as well as the large variations observed in the XF n molecules. We also consider the significant parallels that exist between the XF −1–F bond energies. In addition, the formation of recoupled pair bonds leads to the presence of unexpected low-lying excited states in the XF n molecules. We also consider the significant parallels that exist between the XF n families and the YH n families, where Y=Be, B and C. The latter species also involve recoupled pair bonding, which accounts for the divalence of Be, the trivalence of B and the tetravalence of C without invoking the concept of hybridization. [ABSTRACT FROM AUTHOR]

The coupled cluster CCSD(T) method provides a theoretically sound, accurate description of the electronic structure of a wide range of molecules. To obtain accurate results, however, very large basis sets must be used. Since the computational cost of CCSD(T) calculations formally increases with the seventh power of the number of basis functions (N[sup 7]), the CCSD(T) method can only be applied to a restricted range of molecules. In this work we show that the basis set dependence of the CCSD(T) method is well described by perturbation theory. Starting with CCSD(T)/aug-cc-pVTZ calculations, use of the MP3 method to simulate the effect of increasing the basis set to aug-cc-pV5Z leads to average absolute errors, relative to the full CCSD(T)/aug-cc-pV5Z calculations, of less than ±0.4 kcal/mol (D[sub e]), ±0.0002 Å (r[sub e]), ±2 cm[sup -1] (ω[sub e]), 0.1 kcal/mol (IP[sub e]), and 0.2 kcal/mol (EA[sub e]) for the test set of diatomic molecules considered here. Although the corresponding MP2 approximation does not provide this high level of accuracy, it also should be useful for many molecular studies. When properly implemented, the savings in computer time should be significant since the MP3 method formally scales as N[sup 6], while the MP2 method scales as only N[sup 5]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]