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Mixed quantum/classical theory (MQCT) for the treatment of rotationally inelastic transitions during collisions of two identical molecules, described either as indistinguishable or distinguishable partners, is reviewed. The treatment of two molecules as indistinguishable includes symmetrization of rotational wavefunctions, introduces exchange parity, and gives state-to-state transition matrix elements different from those in the straightforward treatment of molecules as distinguishable. Moreover, the treatment of collision partners as indistinguishable is eight times faster. Numerical results, presented for $H_2 + H_2$, CO + CO and $H_2O + H_2O$ systems, indicate good agreement of MQCT calculations with full-quantum calculations from literature and show that an a posteriori correction, applied after the treatment of collision partners as distinguishable, generally produces good results that agree well with the rigorous treatment of collision partners as indistinguishable. This correction for cross section includes either multiplication by 2, or summation over physically indistinguishable processes, depending on the transition type. After this correction, the results of two treatments agree within 5% for most but may reach 10-20% for some transitions. At low collision energies dominated by scattering resonances these differences can be larger, but they tend to decrease as collision energy is increased. It is also shown that if the system is artificially forced to follow the same collision path in the indistinguishable and distinguishable treatments, then all differences between the results of two treatments disappear. This interesting finding gives new insight into the collision process and indicates that indistinguishability of identical collision partners comes into play through the collision path itself, rather than through matrix elements of inelastic transitions.

A new database of collisional rate coefficients for transitions between the rotational states of H$_2$O collided with H$_2$ background gas is developed. The goal is to expand over the other existing databases in terms of the rotational states of water (200 states are included here) and the rotational states of hydrogen (10 states). All four symmetries of ortho and para water combined with ortho, and para hydrogen are considered.The mixed quantum/classical theory of inelastic scattering implemented in the code MQCT is employed. A detailed comparison with previous databases is conducted to ensure that this approximate method is sufficiently accurate. Integration over collision energies, summation over the final states of H$_2$ and averaging over the initial states of H$_2$ is carried out to provide state-to-state, effective, and thermal rate coefficients in a broad range of temperatures.The rate coefficients for collisions with highly excited H$_2$ molecules are presented for the first time. It is found that rate coefficients for rotational transitions in H$_2$O molecules grow with the rotational excitation of H$_2$ projectiles and exceed those of the ground state H$_2$, roughly, by a factor of 2. These data enable more accurate description of water molecules in high-temperature environments, where the hydrogen molecules of background gas are rotationally excited, and the H$_2$O + H$_2$ collision energy is high. The rate coefficients presented here are expected to be accurate up to the temperature of $\sim$ 2000 K.

We developed a general theoretical approach and a user-ready computer code that permit to study the dynamics of collisional energy transfer and ro-vibrational energy exchange in complex molecule-molecule collisions. The method is a mixture of classical and quantum mechanics. The internal ro-vibrational motion of collision partners is treated quantum mechanically using time-dependent Schrodinger equation that captures many quantum phenomena including state quantization and zero-point energy, propensity and selection rules for state-to-state transitions, quantum symmetry and interference phenomena. A significant numerical speed up is obtained by describing the translational motion of collision partners classically, using the Ehrenfest mean-field trajectory approach. Within this framework a family of approximate methods for collision dynamics is developed. Several benchmark studies for diatomic and triatomic molecules, such as H$_2$O and ND$_3$ collided with He, H$_2$ and D$_2$, show that the results of MQCT are in good agreement with full-quantum calculations in a broad range of energies, especially at high collision energies where they become nearly identical to the full quantum results. Numerical efficiency of the method and massive parallelism of the MQCT code permit us to embrace some of the most complicated collisional systems ever studied, such as C$_6$H$_6$ + He, CH$_3$COOH + He and H$_2$O + H$_2$O. Application of MQCT to the collisions of chiral molecules such as CH$_3$CHCH$_2$O + He, and to the molecule-surface collisions is also possible and will be pursued in the future., Comment: This paper is invited to submit to the Faraday's discussion and is accepted

Aim: We present an improved database of temperature dependent rate coefficients for rotational state-to-state transitions in H$_{2}$O + H$_{2}$O collisions. The database includes 231 transitions between the lower $para$- and 210 transitions between the lower $ortho$-states of H$_{2}$O (up to $j=7$) and can be employed for cometary and planetary applications up to the temperature of 1000 K. Methods: New general method is developed and applied which permits to generate rate coefficients for excitation and quenching processes that automatically satisfy the principle of microscopic reversibility and, also, helps to cover the range of low collision energies by interpolation of cross sections between the process threshold and the computed data points. Results: It is found that in the range of intermediate temperatures, $150 < T < 600$ K, new rate coefficients are in good agreement with those reported earlier, but for higher temperatures, $600 < T < 1000$ K, the new revised temperature dependence is recommended. The low temperature range, $5 < T < 150$ K, is now covered, by the abovementioned interpolation of cross sections down to the process threshold.

The extension of mixed quantum/classical theory (MQCT) to describe collisional energy transfer is developed for symmetric-top-rotor + linear-rotor system type and is applied to $ \rm ND_3 + \rm D_2 $. State-to-state transition cross sections are computed in a broad energy range for all possible processes: when both $ \rm ND_3$ and $ \rm D_2$ molecules are excited or both are quenched, when one is excited while the other is quenched and vice versa, when $ \rm ND_3 $ state changes its parity while $ \rm D_2 $ is excited or quenched, and when $ \rm ND_3 $ is excited or quenched while $ \rm D_2 $ remains in the same state, ground or excited. In all these processes the results of MQCT are found to approximately satisfy the principle of microscopic reversibility. For a set of sixteen state-to-state transitions available from literature for collision energy $ \rm 800 cm^{-1} $ the values of cross sections predicted by MQCT are within 8% of accurate full-quantum results. A useful time-dependent insight is obtained by monitoring the evolution of state populations along MQCT trajectories. It is shown that, if before the collision, $ \rm D_2 $ is in its ground state, the excitation of to $ \rm ND_3 $ rotational states proceeds through a two-step mechanism in which the kinetic energy of molecule-molecule collision is first used to excite $ \rm D_2 $ and only then is transferred to the excited rotational states of to $ \rm ND_3 $. It is found that both potential coupling and Coriolis coupling play important roles in $ \rm ND_3 + \rm D_2 $collisions., Comment: Submitted to journal of Physical Chemistry Chemical Physics

In this work we demonstrate a practical prospect of using quantum annealers for simulation of molecular dynamics. A methodology developed for this goal, dubbed Quantum Differential Equations (QDE), is applied to propagate classical trajectories for the vibration of the hydrogen molecule in several regimes: nearly harmonic, highly anharmonic, and dissociative motion. The results obtained using the D-Wave 2000Q quantum annealer are all consistent and quickly converge to the analytical reference solution. Several alternative strategies for such calculations are explored and it was found that the most accurate results and the best efficiency are obtained by combining the quantum annealer with classical post-processing (greedy algorithm). Importantly, the QDE framework developed here is entirely general and can be applied to solve any system of first-order ordinary nonlinear differential equations using a quantum annealer.

A theoretical framework and computer code are developed for accurate calculations of coupled rotational-vibrational states in triatomic molecules using hyper-spherical coordinates and taking into account the Coriolis coupling effect. Concise final formulae are derived for construction of the Hamiltonian matrix using an efficient combination of the FBR and DVR methods with locally optimized basis sets and grids. First, the new code is tested by comparing its results with those of the APH3D program of Kendrick. Then, accurate calculations of the ro-vibrational spectra are carried out for doubly substituted symmetric (18O16O18O) and asymmetric (18O18O16O) ozone isotopomers for total angular momentum up to J=5. Together with similar data recently reported for the singly substituted symmetric (16O18O16O) and asymmetric (16O16O18O) ozone isotopomers, these calculations quantify the role of the Coriolis coupling effect in the large mass-independent isotopic enrichment of ozone, observed in both laboratory experiments and the atmosphere of Earth. It is found that the Coriolis effect in ozone is relatively small, as evidenced by deviations of its rotational constants from the symmetric-top-rotor behavior, by the magnitudes of parity splittings ({\Lambda}-doubling), and by the ratios of ro-vibrational partition functions for asymmetric vs. symmetric ozone molecules. It is concluded that all of these characteristics are influenced by the isotopic masses as much as they are influenced by the overall symmetry of the molecule. It is therefore unlikely that the Coriolis coupling effect could be responsible for symmetry-driven mass-independent fractionation of oxygen isotopes in ozone.

Several alternative methods for description of interaction between rotation and vibration are compared and contrasted using hyper-spherical coordinates for a triatomic molecule. These methods differ by the choice of z-axis and by the assumption of a prolate or oblate rotor shape of the molecule. For each case, block-structure of the rotational-vibrational Hamiltonian matrix is derived and analyzed, and the advantages and disadvantages of each method are made explicit. This theory is then employed to compute ro-vibrational spectra of singly substituted ozone; roughly, 600 vibrational states of 16O18O16O and 16O16O18O isomers combined, with rotational excitations up to J=5 and both inversion parities (21600 coupled ro-vibrational states total). Splittings between the states of different parities, so called K-doublings, are calculated and analyzed. The roles of the asymmetric-top rotor term and the Coriolis coupling term are determined individually, and it is found that they both affect these splittings, but in the opposite directions. Thus, the two effects partially cancel out, and the residual splittings are relatively small. Energies of the ro-vibrational states reported in this work for 16O18O16O and 16O16O18O are in excellent agreement with literature (available for low vibrational excitation). New data obtained here for the highly excited vibrational states enable the first systematic study of the Coriolis effect in symmetric and asymmetric isotopomers of ozone.

Quantum computers are ideal for solving chemistry problems due to their polynomial scaling with system size in contrast to classical computers which scale exponentially. Until now molecular energy calculations using quantum computing hardware have been limited to quantum simulators. In this paper, a new methodology is presented to calculate the vibrational spectrum of a molecule on a quantum annealer. The key idea of the method is a mapping of the ground state variational problem onto an Ising or quadratic unconstrained binary optimization (QUBO) problem by expressing the expansion coefficients using spins or qubits. The algorithm is general and represents a new revolutionary approach for solving the real symmetric eigenvalue problem on a quantum annealer. The method is applied to two chemically important molecules: O$_2$ (oxygen) and O$_3$ (ozone). The lowest two vibrational states of these molecules are computed using both a hardware quantum annealer and a software based classical annealer., Comment: 29 pages, 11 figures