David E. Woon

Gaussian basis sets for use in correlated molecular calculations. V. Core‐valence basis sets for boron through neon

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...

Gaussian basis sets for use in correlated molecular calculations. IV. Calculation of static electrical response properties

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.

Benchmark calculations with correlated molecular wave functions. IV. The classical barrier height of the H+H2→H2+H reaction

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.

Benchmark calculations with correlated molecular wave functions. I: Multireference configuration interaction calculations for the second row diatomic hydrides

Multireference configuration interaction calculations (valence electrons only) based on generalized valence bond (GVB) and complete active space (CAS) self‐consistent field wave functions are used to compute potential energy functions and spectroscopic constants for the second row diatomic hydrides of aluminum through chlorine. The correlation consistent basis sets of Dunning and co‐workers have been used. This suite of sets—standard and augmented sets of double through quintuple zeta quality—provides a systematic means of improving the description of chemical bonding. The regularity of De and re as a function of basis set quality allows extrapolation to an estimated ‘‘complete’’ basis set limit. The error in the CAS+1+2 predictions of De for the five species varies from 0.3 kcal/mol (AlH) to 1.4 kcal/mol (HCl) with a root‐mean‐square (rms) error of 0.7 kcal/mol. The error in re varies from 0.0008 A (SH) to 0.0028 A (SiH) with a rms error of 0.002 A. Other properties are described with comparable accuracy...

An extended basis set ab initio study of alkali metal cation–water clusters

Ionic clusters comprised of a single alkali metal cation and up to eight water molecules were studied at the Hartree–Fock and correlated levels of theory using the correlation consistent sequence of basis sets. Estimates of the degree of convergence in the computed properties with respect to the complete basis set limit were facilitated by the underlying systematic manner in which the correlation consistent sets approach completeness. In favorable cases, improved property values could be obtained by fitting finite basis set results with a simple analytical expression in order to extrapolate to the complete basis set limit. The sensitivity of structures and binding energies were analyzed with regard to the inclusion of valence and core‐valence correlation recovery at the MP2, MP4, and CCSD(T) levels of theory. The replacement of metal core electrons and the introduction of relativistic contributions via effective core potentials was compared to corresponding all‐electron results.

Benchmark calculations with correlated molecular wave functions. V. The determination of accurate abinitio intermolecular potentials for He2, Ne2, and Ar2

Dimer interactions of helium, neon, and argon have been studied using the augmented correlation consistent basis sets of Dunning and co‐workers. Two correlation methods have been employed throughout; Mo/ller–Plesset perturbation theory through fourth‐order (MP4) and single and double excitation coupled‐cluster theory with perturbative treatment of triple excitations [CCSD(T)]. Full configuration interaction (FCI) calculations were performed on He2 for some basis sets. In general, only valence electrons were correlated, although some calculations which also correlated the n=2 shell of Ar2 were performed. Dimer potential energy curves were determined using the supermolecule method with and without the counterpoise correction. A series of additional basis sets beyond the augmented correlation consistent sets were explored in which the diffuse region of the radial function space has been systematically saturated. In combination with the systematic expansion across angular function space which is inherent to t...

Benchmark calculations with correlated molecular wave functions. VI. Second row A2 and first row/second row AB diatomic molecules

Benchmark calculations employing the correlation consistent basis sets of Dunning and co‐workers are reported for the following diatomic species: Al2, Si2, P2, S2, Cl2, SiS, PS, PN, PO, and SO. Internally contracted multireference configuration interaction (CMRCI) calculations (correlating valence electrons only) have been performed for each species. For Cl2, P2, and PN, calculations have also been carried out using Mo/ller–Plesset perturbation theory (MP2, MP3, MP4) and the singles and doubles coupled‐cluster method with and without perturbative triples [CCSD, CCSD(T)]. Spectroscopic constants and dissociation energies are reported for the ground state of each species. In addition, the low‐lying excited states of Al2 and Si2 have been investigated. Estimated complete basis set (CBS) limits for the dissociation energies, De, and other spectroscopic constants are obtained from simple exponential extrapolations of the computed quantities. At the CBS limit the root‐mean‐square (rms) error in De for the CMRCI...

A CORRELATED AB INITIO STUDY OF LINEAR CARBON-CHAIN RADICALS CNH (N=2-7)

Linear carbon-chain radicals CnH for n = 2-7 have been studied with correlation consistent valence and core-valence basis sets and the coupled cluster method RCCSD(T). Equilibrium structures, rotational constants, and dipole moments are reported and compared with available experimental data. The ground state of the even-n series changes from 2 sigma+ to 2 pi as the chain is extended. For C4H, the 2 sigma+ state was found to lie only 72 cm-1 below the 2 pi state in the estimated complete basis set limit for valence correlation. The C2H- and C3H- anions have also been characterized.

Accurate modeling of intermolecular forces: a systematic Møller-Plesset study of the argon dimer using correlation consistent basis sets

Abstract The argon dimer potential energy curve has been investigated using Moller-Plesset perturbation theory through fourth-order (MP4) and the recently developed augmented correlation consistent basis sets for argon. The interaction energy has been determined using the supermolecule approach; trends in both uncorrected and counterpoise-corrected values are examined. The best corrected values attained for the well depth and equilibrium separation are, respectively, 0.42 mEh and 7.17 a0, which are in very good agreement with the empirical values of 0.45 mEh and 7.10 a0. Moreover, the regularity of the trends for the four basis sets used allows estimates of the complete basis set MP4 values, 0.45 mEh and 7.13 a0.

The Effect of Basis Set Superposition Error (BSSE) on the Convergence of Molecular Properties Calculated with the Correlation Consistent Basis Sets

It is shown that, in many cases, the convergence behavior of molecular properties computed with the correlation consistent basis sets (both standard and augmented sets) is significantly improved if basis set superposition error (BSSE) is taken into account. The effects are most pronounced for pure van der Waals systems like the helium or argon dimers. For these systems the uncorrected D e , r e , and ω e behave very irregularly with increasing basis set size, with the convergence behavior being dramatically improved by use of the counterpoise procedure. Even for strongly bound diatomics like N 2 , HF, and HCl, the counterpoise correction often significantly improves the convergence behavior of r e and ω e . Similar behavior is observed in the weakly bound molecular complexes, ArHF, HCO − , and (HF) 2 , as well as for the more strongly bound HCO molecule. For HCO − , because of the pronounced lengthening of the CO bond upon molecular formation, the deformation energy must also be taken into account.