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Register a new userRepresenting Molecular Ground and Excited Vibrational Eigenstates with Nuclear Densities obtained from Semiclassical Initial Value Representation Molecular Dynamics
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Added by
Gianluca Bertaina Dr
Publication date
2020
fields
Physics
and research's language is
English
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We present in detail and validate an effective Monte Carlo approach for the calculation of the nuclear vibrational densities via integration of molecular eigenfunctions that we have preliminary employed to calculate the densities of the ground and the excited OH stretch vibrational states in protonated glycine molecule [C. Aieta et. al. Nat. Commun. 11, 4348 (2020)]. Here, we first validate and discuss in detail the features of the method on a benchmark water molecule. Then, we apply it to calculate on-the-fly the ab initio anharmonic nuclear densities in correspondence of the fundamental transitions of NH and CH stretches in protonated glycine. We show how we can gain both qualitative and quantitative physical insight by inspection of different one-nucleus densities and assign a character to spectroscopic absorption peaks using the expansion of vibrational states in terms of harmonic basis functions. The visualization of the nuclear vibrations in a purely quantum picture allows us to observe and quantify the effects of anharmonicity on the molecular structure, and to exploit the effect of IR excitations on specific bonds or functional groups, beyond the harmonic approximation. We also calculate the quantum probability distribution of bond-lengths, angles and dihedrals of the molecule. Notably, we observe how in the case of one type of fundamental NH stretching the typical harmonic nodal pattern is absent in the anharmonic distribution.
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A method for carrying out semiclassical initial value representation calculations using first-principles molecular dynamics (FP-SC-IVR) is presented. This method can extract the full vibrational power spectrum of carbon dioxide from a single trajectory providing numerical results that agree with experiment even for Fermi resonant states. The computational demands of the method are comparable to those of classical single-trajectory calculations, while describing uniquely quantum features such as the zero-point energy and Fermi resonances. By propagating the nuclear degrees of freedom using first-principles Born-Oppenheimer molecular dynamics, the stability of the method presented is improved considerably when compared to dynamics carried out using fitted potential energy surfaces and numerical derivatives.
We extend the Mixed Quantum-Classical Initial Value Representation (MQC-IVR), a semiclassical method for computing real-time correlation functions, to electronically nonadiabatic systems using the Meyer-Miller-Stock-Thoss (MMST) Hamiltonian to treat electronic and nuclear degrees of freedom (dofs) within a consistent dynamic framework. We introduce an efficient symplectic integration scheme, the MInt algorithm, for numerical time-evolution of the nuclear and electronic phase space variables as well as the Monodromy matrix, under the non-separable MMST Hamiltonian. We then calculate the probability of transmission through a curve-crossing in model two-level systems and show that in the quantum limit MQC-IVR is in good agreement with the exact quantum results, whereas in the classical limit the method yields results in keeping with mean-field approaches like the Linearized Semiclassical IVR. Finally, exploiting the ability of MQC-IVR to quantize different dofs to different extents, we present a detailed study of the extents to which quantizing the nuclear and electronic dofs improves numerical convergence properties without significant loss of accuracy.
For a small fraction of hot CO2 molecules immersed in a liquid-phase CO2 thermal bath, classical cavity molecular dynamics simulations show that forming collective vibrational strong coupling (VSC) between the C=O asymmetric stretch of CO2 molecules and a cavity mode accelerates hot-molecule relaxation. The physical mechanism underlying this acceleration is the fact that polaritons, especially the lower polariton, can be transiently excited during the nonequilibrium process, which facilitates intermolecular vibrational energy transfer. The VSC effects on these rates (i) resonantly depend on the cavity mode detuning, (ii) cooperatively depend on molecular concentration or Rabi splitting, and (iii) collectively scale with the number of hot molecules, which is similar to Dickes superradiance. For larger cavity volumes, due to a balance between this superradiant-like behavior and a smaller light-matter coupling, the total VSC effect on relaxation rates can scale slower than $1/N$, and the average VSC effect per molecule can remain meaningful for up to $N sim10^4$ molecules forming VSC. Moreover, we find that the transiently excited lower polariton prefers to relax by transferring its energy to the tail of the molecular energy distribution rather than equally distributing it to all thermal molecules. Finally, we highlight the similarities of parameter dependence between the current finding with VSC catalysis observed in Fabry-Perot microcavities.
We show that the centroid molecular dynamics (CMD) method provides a realistic way to calculate the thermal diffusivity $a=lambda/rho c_{rm V}$ of a quantum mechanical liquid such as para-hydrogen. Once $a$ has been calculated, the thermal conductivity can be obtained from $lambda=rho c_{rm V}a$, where $rho$ is the density of the liquid and $c_{rm V}$ is the constant-volume heat capacity. The use of this formula requires an accurate quantum mechanical heat capacity $c_{rm V}$, which can be obtained from a path integral molecular dynamics simulation. The thermal diffusivity can be calculated either from the decay of the equilibrium density fluctuations in the liquid or by using the Green-Kubo relation to calculate the CMD approximation to $lambda$ and then dividing this by the corresponding approximation to $rho c_{rm V}$. We show that both approaches give the same results for liquid para-hydrogen and that these results are in good agreement with experimental measurements of the thermal conductivity over a wide temperature range. In particular, they correctly predict a decrease in the thermal conductivity at low temperatures -- an effect that stems from the decrease in the quantum mechanical heat capacity and has eluded previous para-hydrogen simulations. We also show that the method gives equally good agreement with experimental measurements for the thermal conductivity of normal liquid helium.
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