Thorn H. Dunning

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.

Gaussian basis sets for use in correlated molecular calculations. IX. The atoms gallium through krypton

Valence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions (although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms). After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: cc-pVDZ: (14s11p6d)/[5s4p2d]; cc-pVTZ: (20s13p9d1f )/[6s5p3d1f]; cc-pVQZ: (21s16p12d2 f1g)/[7s6p4d2 f1g]; cc-pV5Z: (26s17p13d3f2g1h)/[8s7p5d3f2g1h]. Augmented sets were obtained by adding diffuse functions to the above sets (one for each angular momentum present in th...

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...

Theory of Hypervalency: Recoupled Pair Bonding in SFn (n = 1-6)

To gain new insight into the nature of hypervalency, we have characterized the bonding across the entire SF(n) sequence (n = 1-6) with high-level quantum chemical theory (multireference configuration interaction and coupled cluster calculations using correlation consistent basis sets). In contrast to most previous studies, this work examined both the stable equilibrium structures and the process of SF(n)-F bond formation. We conclude that two different types of bonding can occur in these species: normal polar covalent bonding and a new type that we call recoupled pair bonding. The two bonding processes can be seen in diatomic SF, where hypervalent behavior first occurs. In the covalently bonded (2)Pi ground state, the bond is formed by straightforward singlet coupling of electrons in the singly occupied S 3p and F 2p orbitals. But there is also a low-lying (4)Sigma(-) excited state where the S 3p(2) pair of electrons must first be decoupled so that one of the electrons can singlet couple with the electron in the F 2p orbital, hence the term recoupled pair bonding. Energy is required to decouple the electron pair, but the bond energy of SF((4)Sigma(-)) is still a substantial fraction (about 40%) of the bond energy of SF((2)Pi). Recoupled pair bonding is the basis for hypervalent behavior: for example, the three unpaired electrons of SF((4)Sigma(-)) are available for further bond formation, and their spatial orientations clearly anticipate the structure of SF(4). The new model of hypervalent bonding introduced in this work accounts for the observed trends in the structures of SF(n) molecules and the variations in the (SF(n)-F) bond energies. The model also predicts the existence of low-lying excited states in some SF(n) species and provides explanations for their energetic separations and orderings.

Ab initio investigation of the N2–HF complex: Accurate structure and energetics

Augmented correlation consistent basis sets of double (aug‐cc‐pVDZ), triple (aug‐cc‐pVTZ), and modified quadruple zeta (aug‐cc‐pVQZ′) quality have been employed to describe the N2–HF potential energy surface at the Hartree–Fock level and with single reference correlated wave functions including Mo/ller–Plesset perturbation theory (MP2, MP3, MP4) and coupled cluster methods [CCSD, CCSD(T)]. The most accurate computed equilibrium binding energies De are (with counterpoise correction) 810 cm−1 (MP4/aug‐cc‐pVQZ′) and 788 cm−1 [CCSD(T)/aug‐cc‐pVQZ′]. Estimated complete basis set limits of 814 cm−1 (MP4) and 793 cm−1 [CCSD(T)] indicate that the large basis set results are essentially converged. Harmonic frequencies and zero‐point energies were determined through the aug‐cc‐pVTZ level. Combining the zero point energies computed at the aug‐cc‐pVTZ level with the equilibrium binding energies computed at the aug‐cc‐pVQZ′ level, we predict D0 values of 322 and 296 cm−1, respectively, at the MP4 and CCSD(T) levels of...

The electronic structure of vanadium carbide, VC

Within an energy range of 2.4 eV, we have explored 29 of the 36 states of the diatomic molecule VC that arise from the atoms in their ground state, V(4s23d3;4F)+C(2s2 2p2;3P). We use multireference methods with large atomic natural orbital basis sets. The ground state is of 2Delta symmetry with the first two excited states, 4Delta and 2Sigma+, located 4.2 and 7.0 kcal/mol above the X state. All the states examined in this work are relatively strongly bound and show significant charge transfer from V to C. The binding energy of the X 2Delta state is estimated to be 95.3 kcal/mol in good agreement with the experimental value.

Bonding in SCln (n = 1-6): a quantum chemical study.

Following a previous study of bonding and isomerism in the SF(n) and singly chloro-substituted SF(n-1)Cl (n = 1-6) series, we describe bonding in the ground and low-lying excited states of the completely substituted series, SCl(n) (n = 1-6). All structures were characterized at least at the RCCSD(T)/aug-cc-pV(Q+d)Z level of theory. Both differences and similarities were observed between SCl(n) and our previous results on SF(n-1)Cl and SF(n). Several minimum structures that exist in SF(n) and SF(n-1)Cl are absent in SCl(n). For example, the optimized structure of SCl(2)((3)A(2)) is a transition state in C(s) symmetry, whereas the analogous states are minima in SF(n) and SF(n-1)Cl. Second, we found a continuation of a trend discovered in the SF(n-1)Cl series, where Cl substitution has a destabilizing effect that weakens bonds with respect to SF(n). This effect is much stronger in the SCl(n) series than it is in the SF(n-1)Cl series, which is why SCl(2) is the most stable observed species in the family and why SCl(4), SCl(5), and SCl(6) are unstable (SCl(n-2) + Cl(2) additions are endothermic for n = 4-6).

First principles investigation of chromium carbide, CrC

We have investigated the electronic structure of 14 states of the experimentally unknown diatomic molecule chromium carbide, CrC, using standard multireference configuration interaction methods and high quality basis sets. We report potential curves, binding energies, and a number of spectroscopic parameters. The ground state of CrC, X 3Sigma-, displays triple-bond character with a binding energy of D(e)=89 kcal/mol and an internuclear separation of r(e)=1.63 A. The first excited state (1 5Sigma-) lies 9.2 kcal/mol higher. All the states studied are fairly ionic, featuring an electron transfer of 0.3-0.5e- from the metal atom to the carbon atom.

Ab initio study of the electronic structure of manganese carbide

We report electronic structure calculations on 13 states of the experimentally unknown manganese carbide (MnC) using standard multireference configuration interaction (MRCI) methods coupled with high quality basis sets. For all states considered we have constructed full potential energy curves and calculated zero point energies. The X state, correlating to ground state atoms, is of 4sigma- symmetry featuring three bonds, with a recommended dissociation energy of D0 = 70.0 kcal/mol and r(e) = 1.640 angstroms. The first and second excited states, which also correlate to ground state atoms, are of 6sigma- and 8sigma- symmetry, respectively, and lie 17.7 and 28.2 kcal/mol above the X state at the MRCI level of theory.

The electronic structure of the two lowest states of CuC.

State-of-the-art ab initio quantum mechanical methods and large basis sets are employed for the study of the electronic structure of the first two states of CuC, (4)Sigma(-) and (2)Pi. A one-electron sigma bond state ((4)Sigma(-)) competes with a two-electron sigma-bond state ((2)Pi) for the ground state of the CuC system. The combined effects of core-valence correlation and relativity point to an X-state of (2)Pi symmetry with D(e)=51.9 kcal/mol and r(e)=1.772 A. The (4)Sigma(-) state is predicted to lie 2.1 kcal/mol higher at r(e)=1.787 A.