Influence of basis set on the calculated properties of (H3N–HCl)

The structure of (H3N–HCl) is investigated by ab initio calculations using a number of different basis sets ranging from minimal to split valence. The effects of including a diffuse sp shell and d orbitals on Cl are considered as well. The geometries of the complex and the isolated subunits are fully optimized. Minimal basis sets (STO‐3G, STO‐6G, and MINI‐1) lead to an overestimate of the interaction between the subunits. Addition of d functions produces only a marginal improvement. The 3‐21G, 3‐21+G, MIDI‐1, and LP‐31G split‐valence sets erroneously predict an ion pair (H3NH+⋅⋅⋅−Cl) in the equilibrium structure, a conclusion which is reversed by polarization of each basis. On the other hand, both the ion pair and (H3N⋅⋅⋅HCl) complexes are identified as minima in the 4‐31G potential. When this basis set is augmented with d functions, agreement with previous calculations involving large basis sets is quite good

The Potential Energy Surface of (NH3)2

Ab initio calculations at the SCF and correlated levels are carried out to characterize the potential energy surface of the NH3 dimer. The two basis sets used are 4‐31G∗ and a larger one containing two sets of d‐functions on N centers, 6‐31G∗∗ (1p, 2d). The only minimum occurring on the surface is a cyclic C2h structure in which the two H‐bonding protons are displaced 42° from the N‐‐N axis. The surface contains a very shallow valley along the direction leading from this geometry to a single linear H bond although there is no minimum corresponding to this arrangement. Despite the symmetrically nonpolar character of the cyclic geometry, the shallowness of the potential in the direction of the linear structure may allow zero point vibration effects to displace the minimum, thereby leading to the observed small dipole moment and its sensitivity to isotopic substitution

Primary and secondary basis set superposition error at the SCF and MP2 levels. H3N‐‐Li+ and H2O‐‐Li+

The primary basis set superposition error (BSSE) results from the artificial lowering of the energy of each subunit of a pair by the presence of ‘‘ghost orbitals’’ of its partner. In addition, these ghost orbitals perturb the one‐electron properties of the molecule, causing a change in the interaction energy, an effect known as secondary BSSE which is not corrected by the counterpoise procedure. The primary and secondary BSSE are calculated for the interactions of NH3 and H2O with Li+, using a variety of different basis sets. It is found that the 2° BSSE can be quite large, comparable in magnitude to the 1° component at both the SCF and MP2 levels. There is no basis found for the supposition that 2° BSSE improves the calculated interaction energy, nor do the 1° and 2° effects cancel one another in general. While the MP2 BSSE tends to be smaller than the SCF analog, the former can be similar in magnitude to the ‘‘true’’ MP2 contribution to the interaction; failure to remove the BSSE can hence lead to a qualitatively incorrect interpretation of the effects of electron correlation. Comparison with a system in which basis set superposition is rigorously excluded suggests that subtraction of both the full 1° and 2° BSSE is appropriate and does not overcorrect the potential. Addition of a diffuse sp shell, especially if coupled with orbital exponent reoptimization, leads to a lowering of the 1° and 2° BSSE, which moreover take on opposite sign and cancel one another to some extent

Ab initio study of FH–PH3 and ClH–PH3 including the effects of electron correlation

Ab initio calculations are carried out for FH–PH3 and ClH–PH3 using a basis set including two sets of polarization functions. Electron correlation is incorporated via Møller–Plesset perturbation theory to second and (in part) to third orders. The basis set is tested and found to produce satisfactory treatments of subsystem properties including geometries and dipole moments as well as the proton affinity and inversion barrier of PH3. Electron correlation is observed to markedly enhance the interaction between PH3 and the hydrogen halides. Its contribution to the complexation energy is 30% for FH–PH3 and 50% for ClH–PH3. Moreover, the equilibrium geometries of the complexes at correlated levels are quite different than SCF structures

Structure, energetics, and vibrational spectrum of H2O–HCl

H2O–HCl is studied using a number of basis sets including 6‐31G∗∗ and variants which are augmented by a diffuse sp shell and a second set of d functions on O and Cl. Optimization of the geometry of the complex is carried out including explicitly electron correlation and counterpoise correction of the basis set superposition error (BSSE) at both the SCF and correlated levels. Correlation strengthens and shortens the H bond while BSSE correction leads to an opposite trend; these two effects are of different magnitude and hence cancel one another only partially. ΔH°(298 K) is calculated to be −4.0 kcal/mol, 1/4 of which is due to correlation. Formation of the complex causes the strong intensification and red shift of the H–Cl stretching band normally associated with H bonding, whereas the internal vibrations of H2O are very little affected, except for a doubling of the intensity of the symmetric stretch. With respect to the intermolecular modes, the bends of the proton donor are of higher frequency than those involving the acceptor. While these intermolecular bends are all of moderate intensity, comparable to the intramolecular modes, the H‐bond stretch νσ is very weak indeed, consistent with a principle involving subunit dipoles. All calculated vibrational data are in excellent agreement with the spectra measured in solid inert gas matrices

Energetics of proton transfer between carbon atoms (H3CH [BOND] CH3)−

Ab initio calculations were carried out to study the potential energy surface of (H3CHCH3)−. The 6–31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH4 and CH3− to form the (H3CHCH3)− complex is about 2 kcal/mol, much smaller than for comparable ionic H-bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller-Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13‒15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions

Effects of basis set and electron correlation on the calculated properties of the ammonia dimer

Ab initio calculations are carried out for (NH3)2 with a 6‐31G∗∗(1p,2d) basis set containing diffuse polarization functions. Electron correlation is included via second‐order Møller–Plesset perturbation theory (MP2). At the SCF level, the equilibrium R(NN) distance is 3.54 Å and the interaction energy is −2.35 kcal/mol. Inclusion of correlation enhances the attraction substantially, increasing the energy to −4.05 kcal/mol and reducing the intermolecular separation by 0.20 Å. Comparison with previous results at the SCF level demonstrates a variety of errors including exaggerated dipole moments, underestimation of polarization energy, and sizable superposition errors with these smaller basis sets

Correlated Proton Transfer Potentials. (HO-H-OH)- and (H2O-H-OH2)+

Potentials are computed for the transfer of a proton between two hydroxide anions in (HO-H-OH)− and between two neutral water molecules in (H2O-H-OH2)+ using polarized basis sets of various sizes. SCF transfer barriers are lowered in both complexes by electron correlation. As progressively higher orders of Moller-Plesset perturbation theory are applied, the correction reverses sign and diminishes in magnitude. As a consequence, MP2 results nearly coincide with full fourth-order data. The highest level calculations indicate the transfer barrier of (HO-H-OH)− exceeds that of (H2O-H-OH2)+ by 20–30%. The MP4 calculations agree well with coupled cluster CCD + ST results which include single, double, and triple substitutions to high order

Proton Transfer in the Ground and First Excited Triplet States of Malonaldehyde

The intramolecular proton-transfer process in malonaldehyde is studied by ab initio methods in its ground electronic state and in its excited r - r* triplet state. The transfer barrier is considerably higher in T1, which is attributed in part to the virtual disappearance of the hydrogen-bonding interaction present in the ground state. The weakening of this H bond in the excited state is associated with a less acidic protondonor OH group and less basic acceptor oxygen. The r - r* excitation adds antibonding character to the C=O bond. which causes it to elongate. The ensuing reduction in its bond dipole help to weaken the intramolecular H bond

The Potential Energy Surface and Equilibrium Geometry of Ar··PH3

The potential energy surface of the complex formed between Ar and PH3 is investigated by ab initio methods. In the equilibrium geometry, the Ar atom lies 3.75 Å from the P center; the Ar··P vector makes an angle of 75° with the C3 symmetry axis of PH3. The SCF portion of the interaction energy is repulsive but is counteracted by an attractive MP2 contribution. The SCF term is more sensitive to the relative orientation of the two subunits and is hence responsible for the angular features of the equilibrium structure. The binding energy of Ar··PH3 is calculated to be about 70 cm−1 at the correlated level after correction for basis set superposition error