Ab Initio Interaction Potentials Research Articles

ColdN+NHCollisions in a Magnetic Trap

We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of $\ensuremath{\sim}600\text{ }\text{ }\mathrm{mK}$. We measure a small $\mathrm{N}+\mathrm{NH}$ trap loss rate coefficient of ${k}_{\mathrm{loss}}^{(\mathrm{N}+\mathrm{NH})}=9(5)(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of $\mathrm{N}+\mathrm{NH}$ elastic-to-inelastic collisions remains large ($>100$) into the mK regime.

Collisional properties of cold spin-polarized nitrogen gas: Theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules

We report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of 14N and 15N atoms with densities 5+/-2 x 10^{12} cm-3 and measure their magnetic relaxation rates at milli-Kelvin temperatures. Rigorous quantum scattering calculations based on accurate ab initio interaction potentials for the 7Sigma_u electronic state of N2 demonstrate that dipolar relaxation in N + N collisions occurs at a slow rate of ~10^{-13} cm3/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.

Molecular dynamics study of formamidine decomposition in gas and solution phases via free energy curves from ab initio interaction potentials

A procedure is described for the theoretical study of chemical reactions in solution by means of molecular dynamics simulation, with solute–solvent interaction potentials derived from ab initio quantum calculations. We apply the procedure to the case of the decomposition of formamidine, HCNHNH2 → NH3 + HCN, in both gas and solution phases, via the non-assisted and assisted mechanisms. We used the solvent as reaction coordinate and the free energy curves for the calculation of the activation energies. The results showed that the decomposition assisted with one or two water molecules in aqueous solution is preferred over the non-assisted mechanism, reducing significantly the activation barrier.

Collision-induced spin exchange of alkali-metal atoms with 3He

Collisions of atoms and molecules in external electromagnetic fields can depolarize electronic and nuclear spins, leading to decoherence of quantum superposition states. The fundamental microscopic processes giving rise to spin depolarization are spin exchange and spin relaxation. In this work, we present an analysis of spin exchange transitions in collision of alkali metal atoms with 3He. We use accurate ab initio calculations to evaluate the electron spin density of Li-He and the interaction energy of Rb-He. The Fermi contact interaction constants for the complexes of Na, K, and Rb atoms with 3He are extracted from the measured frequency shift enhancement factors using the ab initio interaction potentials. The spin density calculations also yield the hyperfine pressure shift of Li-He.

Ab initio interaction potentials for X and B excited states of He–I2 for studying dynamics

Ab initio CCSD(T) and MRCI approaches were employed to construct potential energy surfaces of the ground and the B electronic excited states of He–I2 complex, while full quantum mechanical methods were applied to study its spectroscopy and dynamics. A description of the approach adopted, together with the results obtained and their comparison with recent experimental data, as well as further improvements are presented.

Suppression of Zeeman relaxation in cold collisions ofP21/2atoms

We present a combined experimental and theoretical study of angular momentum depolarization in cold collisions of {sup 2}P atoms in the presence of an external magnetic field. We show that collision-induced Zeeman relaxation of Ga({sup 2}P{sub 1/2}) and In({sup 2}P{sub 1/2}) atoms in cold {sup 4}He gas is dramatically suppressed compared to atoms in {sup 2}P{sub 3/2} states. Using rigorous quantum-scattering calculations based on ab initio interaction potentials, we demonstrate that Zeeman transitions in collisions of atoms in {sup 2}P{sub 1/2} electronic states occur via couplings to the {sup 2}P{sub 3/2} state induced by the anisotropy of the interaction potential. Our results suggest the feasibility of sympathetic cooling and magnetic trapping of {sup 2}P{sub 1/2}-state atoms, such as halogens, thereby opening up exciting areas of research in precision spectroscopy and cold-controlled chemistry.

Collision-induced spin exchange of alkali-metal atoms withH3e: Anab initiostudy

We present a rigorous quantum study of spin-exchange transitions in collisions of the alkali-metal atoms with $^{3}\text{H}\text{e}$ in the presence of an external magnetic field. Using accurate ab initio interaction potentials, we obtain refined estimates for the Fermi contact interaction constants for complexes of Na, K, and Rb atoms with $^{3}\text{H}\text{e}$. Ab initio calculations show that the Fermi contact interaction in $\text{Li-}^{3}\text{H}\text{e}$ varies more slowly with internuclear distance than predicted by the atomic model [R. M. Herman, Phys. Rev. 37, A1062 (1965)]. The calculated spin-exchange rate constants for Na, K, and Rb atoms in a gas of $^{3}\text{H}\text{e}$ are in good agreement with experimental data. Our calculations demonstrate that at a temperature of 0.5 K, collision-induced spin exchange of the alkali-metal atoms occurs at a very slow rate of $\ensuremath{\sim}{10}^{\ensuremath{-}22}\text{ }{\text{cm}}^{3}/\text{s}$, suggesting potential applications in cryogenic cooling, precision spectroscopy, and quantum optics.

Quenching efficiency of “hot" polar molecules by He buffer gas at ultralow energies: quantum results for MgH and LiH rotations

Rotationally superelastic collisions are computed from quantum dynamical calculations for the two title systems, down to the very low collision energies of interest in cold trap experiments. The results are obtained via ab initio interaction potentials, the one for the MgH-He system being given here for the first time and discussed in details in the present paper. The calculations show marked differences in behaviour for both cooling efficiency rates at vanishing temperatures and individual cross sections between levels, and their origin is related to target structural properties and with specific features of the two potentials.

Ab initiostudy of Tm-He interactions and dynamics in a magnetic trap

The dynamics of magnetic relaxation of Tm atoms in cold helium gas is investigated using ab initio interaction potentials between Tm({sup 2}F) and He atoms. The interaction of Tm({sup 2}F) with He gives rise to four adiabatic potentials of {sigma}{sup +}, {pi}, {delta}, and {phi} symmetries, which are found to be degenerate to within 0.1 cm{sup -1} at all interatomic distances larger than R=5.4 Angst . The small splitting between the interaction potentials leads to suppression of Zeeman transitions in Tm-He collisions. Our quantum scattering calculations yield a small rate constant of 1.5x10{sup -15} cm{sup 3} s{sup -1} for Zeeman relaxation of the Tm({sup 2}F{sub 7/2}) atoms in the maximally stretched M=7/2 sublevel at the temperature 0.8 K, in reasonable agreement with the measured value (5.0{+-}2.1)x10{sup -15} cm{sup 3} s{sup -1}. The sensitivity of Zeeman-relaxation cross sections to details of the interaction potentials is examined. To our knowledge, this is one of only two comparisons of ab initio quantum scattering calculations with experimental measurements of inelastic collisions at subkelvin temperatures near one Kelvin. Our work thus demonstrates the feasibility of describing cold collisions from first principles.

New model potentials for sulfur–copper(I) and sulfur–mercury(II) interactions in proteins: From ab initio to molecular dynamics

We have developed new force field and parameters for copper(I) and mercury(II) to be used in molecular dynamics simulations of metalloproteins. Parameters have been derived from fitting of ab initio interaction potentials calculated at the MP2 level of theory, and results compared to experimental data when available. Nonbonded parameters for the metals have been calculated from ab initio interaction potentials with TIP3P water. Due to high charge transfer between Cu(I) or Hg(II) and their ligands, the model is restricted to a linear coordination of the metal bonded to two sulfur atoms. The experimentally observed asymmetric distribution of metal ligand bond lengths (r) is accounted for by the addition of an anharmonic (r3) term in the potential. Finally, the new parameters and potential, introduced into the CHARMM force field, are tested in short molecular dynamics simulations of two metal thiolates fragments in water. (Brooks BR et al. J Comput Chem 1983, 4, 1987.1).

Penning ionization electron spectroscopy of C6H6 by collision with He*(2 3S) metastable atoms and classical trajectory calculations: Optimization ofab initiomodel potentials

The potential energy surface of benzene (C(6)H(6)) with a He*(2(3)S) atom was obtained by comparison of experimental data in collision-energy-resolved two-dimensional Penning ionization electron spectroscopy with classical trajectory calculations. The ab initio model interaction potentials for C(6)H(6)+He*(2(3)S) were successfully optimized by the overlap expansion method; the model potentials were effectively modified by correction terms proportional to the overlap integrals between orbitals of the interacting system, C(6)H(6) and He*(2(3)S). Classical trajectory calculations with optimized potentials gave excellent agreement with the observed collision-energy dependence of partial ionization cross sections. Important contributions to corrections were found to be due to interactions between unoccupied molecular orbitals and the He*2s orbital. A C(6)H(6) molecule attracts a He*(2(3)S) atom widely at the region where pi electrons distribute, and the interaction of -80 meV (ca. -1.8 kcal/mol) just cover the carbon hexagon. The binding energy of a C(6)H(6) molecule and a He* atom was 107 meV at a distance of 2.40 A on the sixfold axis from the center of a C(6)H(6) molecule, which is similar to that of C(6)H(6)+Li and is much larger than those of the C(6)H(6)+[He,Ne,Ar] systems.

Study of the stabilization energies of halide-water clusters: an application of first-principles interaction potentials based on a polarizable and flexible model.

The aim of this work is to compute the stabilization energy E(stab)(n) of [X(H(2)O)(n)](-) (X identical with F, Br, and I for n=1-60) clusters from Monte Carlo simulations using first-principles ab initio potentials. Stabilization energy of [X(H(2)O)(n)](-) clusters is defined as the difference between the vertical photodeachment energy of the cluster and the electron affinity of the isolated halide. On one hand, a study about the relation between cluster structure and the E(stab)(n) value, as well as the dependence of the latter with temperature is performed, on the other hand, a test on the reliability of our recently developed first-principles halide ion-water interaction potentials is carried out. Two different approximations were applied: (1) the Koopmans' theorem and (2) calculation of the difference between the interaction energy of [X(H(2)O)(n)](-) and [X(H(2)O)(n)] clusters using the same ab initio interaction potentials. The developed methodology allows for using the same interaction potentials in the case of the ionic and neutral clusters with the proviso that the charge of the halide anion was switched off in the latter. That is, no specific parametrization of the interaction potentials to fit the magnitude under study was done. The good agreement between our predicted E(stab)(n) and experimental data allows us to validate the first-principles interaction potentials developed elsewhere and used in this study, and supports the fact that this magnitude is mainly determined by electrostatic factors, which can be described by our interaction potentials. No relation between the value of E(stab)(n) and the structure of clusters has been found. The diversity of E(stab)(n) values found for different clusters with similar interaction energy indicates the need for statistical information to properly estimate the stabilization energy of the halide anions. The effect of temperature in the prediction of the E(stab)(n) is not significant as long as it was high enough to avoid cluster trapping into local equilibrium configurations which guarantees an appropriate sampling of the configurational space. Parallel tempering method was applied in particular cases to guarantee satisfactory sampling of clusters at low temperature.

Solvation of triplet Rydberg states of molecular hydrogen in superfluid helium

We report ab initio interaction potentials, transition dipole moments, and radiative lifetimes for the four lowest triplet states of ${\mathrm{H}}_{2}:$ $b$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ $c$ ${}^{3}{\ensuremath{\Pi}}_{u},$ $a$ ${}^{3}{\ensuremath{\Sigma}}_{g}^{+},$ and $e$ ${}^{3}{\ensuremath{\Sigma}}_{u}^{+},$ and their response to the perturbation due to approaching ground state He atom. Hybrid density functional--quantum Monte Carlo calculations employing the ab initio interaction potentials are then used for calculating the liquid structure around the molecular excimers in bulk superfluid ${}^{4}\mathrm{He}.$ Calculations demonstrate a wide variety of possible solvation structures, both spherical and highly anisotropic in geometry, depending on the electronic state of ${\mathrm{H}}_{2}.$ The experimentally observed ${\mathrm{H}}_{2}$ ${(}^{3}{e}^{3}a)$ emission bands [Trottier et al., Phys. Rev. A 61, 052504 (2000)] are simulated and the origins of the line shifts discussed. Absorption spectra of the same system are predicted to be broader and more blue shifted compared to the gas phase. Feasibility of the metastable ${}^{3}$$c$ state for absorption experiments in liquid helium is proposed.

Rapid Calculation of the Structures of Solutions with ab Initio Interaction Potentials

A non-Boltzmann sampling method is demonstrated for efficient calculation of structural properties of models having ab initio interaction potentials. A sample of independent configurations is generated with a molecular dynamics simulation using an approximate potential. Ab initio calculations, done only at the sampled configurations, are used to correct the thermal averages of any mechanical variable for the difference between the approximate and ab initio potentials. Tests of the method show that relatively few ab initio calculations are needed to obtain significant structural information and that bootstrap estimates of the uncertainties are reasonable. As a further test, we use results from a previous calculation of the hydration free energy of Na + at 973 K and 0.535 g/cm 3 to calculate the Na-O pair correlation function and coordination number for an ab initio model of Na + -H 2 O interactions.

Theoretical prediction of the ArO − anion ZEKE photoelectron spectrum

High-resolution zero electron kinetic energy spectra of the weakly bound ArO − anion are predicted using the method which combines atoms-in-molecule and Rau–Fano models for calculating relative intensities and accurate ab initio interaction potentials. ZEKE spectrum envelope and its temperature variation are shown to be essentially determined by the electronic photodetachment probabilities which are highly selective with respect to the symmetry of the spin–orbit components of anion and neutral electronic states.

Ab Initio Interaction Potentials for Simulations of Dimethylnitramine Solutions in Supercritical Carbon Dioxide with Cosolvents

Symmetry-adapted perturbation theory has been employed to calculate ab initio potential energy surfaces for complexes involved in the process of dissolving dimethylnitramine in supercritical carbon dioxide in the presence of acetonitrile or methyl alcohol as cosolvents. Site−site fits with correct asymptotic behavior have been developed for all potentials. The new potential energy surfaces can be used, along with the previously reported ones for carbon dioxide and carbon dioxide−acetonitrile dimers, in fully ab initio simulations of supercritical extraction processes. The physical interpretation of the features of the interaction potential energy surfaces resulting from the present approach challenges the established literature interpretations in terms of electrostatics only.

First-Principles Ion−Water Interaction Potentials for Highly Charged Monatomic Cations. Computer Simulations of Al3+, Mg2+, and Be2+in Water

The concept of hydrated ion [M(H2O)m ]n+ has been used to describe interactions of highly charged monatomic cations in water. Ab initio interaction potentials for Al3+, Mg2+, and Be2+ have been developed on the basis of that previously applied to the Cr3+ hydration study (Martinez et al. J. Chem. Phys. 1998, 109, 1445). Transferability of the different contributions to the intermolecular potentials that describe the interactions between the central cation and the first hydration shell, Mn+−(H2O)I, and the hydrate with bulk water molecules, [M(H2O)m ]n+−(H2O)bulk, have been examined. Results indicate that a reduced number of points (∼40 quantum chemical computations) are enough to get the basic and differential features of each cation. MD simulations (200 ps) for the hydrate of each cation plus 512 TIP4P H2O have been performed. Structural properties such as RDFs among different pairs of atoms in the system and orientational parameter are presented. Likewise, dynamical properties such as self-diffusion coe...

Natural Energy Decomposition Analysis: The Linear Response Electrical Self Energy

An extension of the natural energy decomposition analysis (NEDA) is described that leads to a reduced, three-component treatment of ab initio molecular interaction potentials. These components include the classical electrical (static and induced) and quantum mechanical core (σ−σ) and charge transfer (σ−σ*) interactions. The electrical component is, in turn, represented by three terms: the static interaction of unperturbed fragment charge densities, a contribution involving polarized charge densities, and an electrical self polarization energy. Extended basis set calculations demonstrate the high numerical stability of the three-component NEDA approach. Applications are presented for the Li+(H2O) and water dimer complexes. The long-range behavior of the interaction potentials of these complexes is entirely determined by the classical electrical interaction of the fragments. Core and charge transfer effects are only significant for molecular separations within roughly 1 A of equilibrium.

Free energies and structures of hydrated cations, based on effective pair potentials

We present a method, based on a continuum representation of the solvent, to compute ab initio effective interaction potentials for solvated pairs. Such potentials take into account many-body effects, thus overcoming the non-additivity errors affecting uncorrected pair potentials. We apply the method to cation-water interactions, for a variety of cations: Li+, Be2+, Mg2+, Ca2+, Ni2+, Zn2+ and Al3+. The potentials thus obtained are suitable for simulations of ionic solutions or clusters of water molecules surrounding a cation. We exploit them to compute hydration free energies ΔGhyd of cations, with the constraint that the first solvation shell contains a given number of water molecules. This enables us to find the thermodynamically most stable solvation number. The effective potential results compare well with experimental values of ΔGhyd and with full ab initio calculations on the [M(H2O)n]q+ complexes.

Ab initio interaction potentials between an Ar atom and the NH radical in the states X 3Σ −, a 1Δ and b 1Σ +

Using extended basis sets in complete active space self-consistent field and averaged coupled-pair functional calculations, the potential energy surfaces for the interaction of an Ar atom with the NH radical in its three lowest electronic states were determined. While the ground-state interaction energy surface was found to have its absolute minimum at a non-linear geometry of the ArNH complex, the other surfaces have their absolute minima at linear geometries with Ar next to H.