Riccardo Conte

Permutationally Invariant Fitting of Many-Body, Non-covalent Interactions with Application to Three-Body Methane-Water-Water.

A modified, computationally efficient method to provide permutationally invariant polynomial bases for molecular energy surface fitting via monomial symmetrization (Xie Z.; Bowman J. M. J. Chem. Theory Comput. 2010, 6, 26-34) is reported for applications to complex systems, characterized by many-body, non-covalent interactions. Two approaches, each able to ensure the asymptotic zero-interaction limit of intrinsic potentials, are presented. They are both based on the tailored selection of a subset of the polynomials of the original basis. A computationally efficient approach exploits reduced permutational invariance and provides a compact fitting basis dependent only on intermolecular distances. We apply the original and new techniques to obtain a number of full-dimensional potentials for the intrinsic three-body methane-water-water interaction by fitting a database made of 22,592 ab initio energies calculated at the MP2-F12 level of theory with haTZ (aug-cc-pVTZ for C and O, cc-pVTZ for H) basis set. An investigation of the effects of permutational symmetry on fitting accuracy and computational costs is reported. Several of the fitted potentials are then employed to evaluate with high accuracy the three-body contribution to the CH4-H2O-H2O binding energy and the three-body energy of three conformers of the CH4@(H2O)20 cluster.

Communication: A benchmark-quality, full-dimensional ab initio potential energy surface for Ar-HOCO

A full-dimensional, global ab initio potential energy surface (PES) for the Ar-HOCO system is presented. The PES consists of a previous intramolecular ab initio PES for HOCO [J. Li, C. Xie, J. Ma, Y. Wang, R. Dawes, D. Xie, J. M. Bowman, and H. Guo, J. Phys. Chem. A 116, 5057 (2012)], plus a new permutationally invariant interaction potential based on fitting 12 432 UCCSD(T)-F12a/aVDZ counterpoise-corrected energies. The latter has a total rms fitting error of about 25 cm−1 for fitted interaction energies up to roughly 12 000 cm−1. Two additional fits are presented. One is a novel very compact permutational invariant representation, which contains terms only involving the Ar-atom distances. The rms fitting error for this fit is 193 cm−1. The other fit is the widely used pairwise one. The pairwise fit to the entire data set has an rms fitting error of 427 cm−1. All of these potentials are used in preliminary classical trajectory calculations of energy transfer with a focus on comparisons with the results u...

Trajectory study of energy transfer and unimolecular dissociation of highly excited allyl with argon.

The influence of rotational excitation on energy transfer in single collisions of allyl with argon and on allyl dissociation is investigated. About 90,000 classical scattering simulations are performed in order to determine collision-induced changes in internal energy and in allyl rotational angular momentum. Dissociation is studied by means of about 50,000 additional trajectories evolved for the isolated allyl under three different conditions: allyl with no angular momentum (J = 0); allyl with the same microcanonically sampled initial conditions used for the collisions (J*); allyl evolving from the corresponding exit conditions after the collision. The potential energy surface is the sum of an intramolecular potential and an interaction one, and it has already been used in a previous work on allyl-argon scattering (Conte, R.; Houston, P. L.; Bowman, J. M. J. Phys. Chem. A 2013, 117, 14028-14041). Energy transfer data show that increased initial rotation favors, on average, increased relaxation of the excited molecule. The availability of a high-level intramolecular potential energy surface permits us to study the dependence of energy transfer on the type of starting allyl isomer. A turning point analysis is presented, and highly efficient collisions are detected. Collision-induced variations in the allyl rotational angular momentum may be quite large and are found to be distributed according to three regimes. The roles of rotational angular momentum, collision, and type of isomer on allyl unimolecular dissociation are considered by looking at dissociations times, kinetic energies of the fragments, and branching ratios. Generally, rotational angular momentum has a strong influence on the dissociation dynamics, while the single collision and the type of starting isomer are less influential.

Classical Trajectory Study of Energy Transfer in Collisions of Highly Excited Allyl Radical with Argon

Predicting the results of collisions of polyatomic molecules with a bath of atoms is a research area that has attracted substantial interest in both experimental and theoretical chemistry. Energy transfer, which is the consequence of such collisions, plays an important role in gas-phase kinetics and relaxation of excited molecules. We present a study of energy transfer in single collisions of highly vibrationally excited allyl radical in argon. We evolve a total of 52 000 classical trajectories on a potential energy surface, which is the sum of an ab initio intramolecular potential for the allyl and a pairwise interaction potential describing the argons effect on the allyl. The former is described by means of a permutationally invariant full-dimensional potential, whereas the interaction potential between allyl and argon is obtained by means of a sum of pairwise potentials dependent on nonlinear parameters that have been fit to a set of MP2/avtz counterpoise corrected ab initio energies. Results are reported for energy transfers and related probability densities at different collisional energies. The sensitivity of results to the interaction potential is considered and the potential is shown to be suitable for future applications involving different isomers of the allyl. The impact of highly efficient collisions in the energy transfer process is examined.

Graphics processing units accelerated semiclassical initial value representation molecular dynamics

This paper presents a Graphics Processing Units (GPUs) implementation of the Semiclassical Initial Value Representation (SC-IVR) propagator for vibrational molecular spectroscopy calculations. The time-averaging formulation of the SC-IVR for power spectrum calculations is employed. Details about the GPU implementation of the semiclassical code are provided. Four molecules with an increasing number of atoms are considered and the GPU-calculated vibrational frequencies perfectly match the benchmark values. The computational time scaling of two GPUs (NVIDIA Tesla C2075 and Kepler K20), respectively, versus two CPUs (Intel Core i5 and Intel Xeon E5-2687W) and the critical issues related to the GPU implementation are discussed. The resulting reduction in computational time and power consumption is significant and semiclassical GPU calculations are shown to be environment friendly.

A novel Gaussian Binning (1GB) analysis of vibrational state distributions in highly excited H2O from reactive quenching of OH* by H2

As shown in experiments by Lester and co-workers [J. Chem. Phys. 110, 11117 (1999)], the reactive quenching of OH∗ by H2 produces highly excited H2O. Previous limited analysis of quasiclassical trajectory calculations using standard Histogram Binning (HB) was reported [B. Fu, E. Kamarchik, and J. M. Bowman, J. Chem. Phys. 133, 164306 (2010)]. Here, we examine the quantized internal state distributions of H2O in more detail, using two versions of Gaussian Binning (denoted 1GB). In addition to the standard version of 1GB, which relies on the harmonic energies of the states (1GB-H), we propose a new and more accurate technique based on exact quantum vibrational energies (1GB-EQ). Data from about 42,000 trajectories from previous calculations that give excited water molecules are used in the two versions of 1GB as well as HB. For the vibrationally hot molecules considered in this study, the classical internal energy distribution serves as a benchmark to estimate the accuracy of the different binning methods analyzed. The 1GB discretization methods, especially the one using exact quantum energies, reconstruct the classical distribution much more accurately than HB and also the original, more elaborate Gaussian Binning method. Detailed quantum state distributions are presented for pure overtone excitations as well as several antisymmetric stretch distributions. The latter are focused on because the antisymmetric stretch has the largest emission oscillator strength of the three water modes.

On-the-Fly ab Initio Semiclassical Calculation of Glycine Vibrational Spectrum

We present an on-the-fly ab initio semiclassical study of vibrational energy levels of glycine, calculated by Fourier transform of the wavepacket correlation function. It is based on a multiple coherent states approach integrated with monodromy matrix regularization for chaotic dynamics. All four lowest-energy glycine conformers are investigated by means of single-trajectory semiclassical spectra obtained upon classical evolution of on-the-fly trajectories with harmonic zero-point energy. For the most stable conformer I, direct dynamics trajectories are also run for each vibrational mode with energy equal to the first harmonic excitation. An analysis of trajectories evolved up to 50 000 atomic time units demonstrates that, in this time span, conformers II and III can be considered as isolated species, while conformers I and IV show a pretty facile interconversion. Therefore, previous perturbative studies based on the assumption of isolated conformers are often reliable but might be not completely appropriate in the case of conformer IV and conformer I for which interconversion occurs promptly.

Collisional energy transfer in highly excited molecules.

The excitation/de-excitation step in the Lindemann mechanism is investigated in detail using model development and classical trajectory studies based on a realistic potential energy surface. The model, based on a soft-sphere/line-of-centers approach and using elements of Landau-Teller theory and phase space theory, correctly predicts most aspects of the joint probability distribution P(ΔE,ΔJ) for the collisional excitation and de-excitation process in the argon-allyl system. The classical trajectories both confirm the validity of the model and provide insight into the energy transfer. The potential employed was based on a previously available ab initio intramolecular potential for the allyl fit to 97418 allyl electronic energies and an intermolecular potential fit to 286 Ar-allyl energies. Intramolecular energies were calculated at the CCSD(T)/AVTZ level of theory, while intermolecular energies were calculated at the MP2/AVTZ level of theory. Trajectories were calculated for each of four starting allyl isomers and for an initial rotational level of Ji = 0 as well as for Ji taken from a microcanonical distribution. Despite a dissimilarity in Ar-allyl potentials for fixed Ar-allyl geometries, energy transfer properties starting from four different isomers were found to be remarkably alike. A contributing factor appears to be that the orientation-averaged potentials are almost identical. The model we have developed suggests that most hydrocarbons should have similar energy transfer properties, scaled by differences in the potential offset of the atom-hydrogen interaction. Available data corroborate this suggestion.

Trajectory and Model Studies of Collisions of Highly Excited Methane with Water Using an ab Initio Potential

Quasi-classical trajectory studies have been performed for the collision of internally excited methane with water using an accurate methane-water potential based on a full-dimensional, permutationally invariant analytical representation of energies calculated at a high level of theory. The results suggest that most energy transfer takes place at impact parameters smaller than about 8 Bohr; collisions at higher impact parameters are mostly elastic. Overall, energy transfer is fairly facile, with values for ⟨ΔEdown⟩ and ⟨ΔEup⟩ approaching almost 2% of the total excitation energy. A classical model previously developed for the collision of internally excited molecules with atoms (Houston, P. L.; Conte, R.; Bowman, J. M. J. Phys. Chem. A 2015, 119, 4695-4710) has been extended to cover collisions of internally excited molecules with other molecules. For high initial rotational levels, the agreement with the trajectory results is quite good (R(2) ≈ 0.9), whereas for low initial rotational levels it is only fair (R(2) ≈ 0.7). Both the model and the trajectories can be characterized by a four-dimensional joint probability distribution, P(J1,f,ΔE1,J2,f,ΔE2), where J1,f and J2,f are the final rotational levels of molecules 1 and 2 and ΔE1 and ΔE2 are the respective changes in internal energy. A strong anticorrelation between ΔE1 and ΔE2 is observed in both the model and trajectory results and can be explained by the model. There is evidence in the trajectory results for a small amount of V ↔ V energy transfer from the water, which has low internal energy, to the methane, which has substantial internal energy. This observation suggests that V ↔ V energy transfer in the other direction also occurs.

Continuum limit frozen Gaussian approximation for the reduced thermal density matrix of dissipative systems

A continuum limit frozen Gaussian approximation is formulated for the reduced thermal density matrix for dissipative systems. The imaginary time dynamics is obtained from a novel generalized Langevin equation for the system coordinates. The method is applied to study the thermal density in a double well potential in the presence of Ohmic-like friction. We find that the approximation describes correctly the delocalization of the density due to quantization of the vibrations in the well. It also accounts for the friction induced reduction of the tunneling density in the barrier region.