Diffusion Monte Carlo (DMC) provides a powerful method for understanding the vibrational landscape of molecules that are not well-described by conventional methods. The most computationally demanding step of these calculations is the evaluation of the potential energy. In this work, a general approach is developed in which a neural network potential energy surface is trained by using data generated from a small-scale DMC calculation. Once trained, the neural network can be evaluated by using highly parallelizable calls to a graphics processing unit (GPU). The power of this approach is demonstrated for DMC simulations on H2O, CH5 +, and (H2O)2. The need to include permutation symmetry in the neural network potentials is explored and incorporated into the molecular descriptors of CH5 + and (H2O)2. It is shown that the zero-point energies and wave functions obtained by using the neural network potentials are nearly identical to the results obtained when using the potential energy surfaces that were used to train the neural networks at a substantial savings in the computational requirements of the simulations.
A machine-learning based approach for evaluating potential energies for quantum mechanical studies of properties of the ground and excited vibrational states of small molecules is developed. This approach uses the molecular-orbital-based machine learning (MOB-ML) method to generate electronic energies with the accuracy of CCSD(T) calculations at the same cost as a Hartree–Fock calculation. To further reduce the computational cost of the potential energy evaluations without sacrificing the CCSD(T) level accuracy, GPU-accelerated Neural Network Potential Energy Surfaces (NN-PES) are trained to geometries and energies that are collected from small-scale Diffusion Monte Carlo (DMC) simulations, which are run using energies evaluated using the MOB-ML model. The combined NN+(MOB-ML) approach is used in variational calculations of the ground and low-lying vibrational excited states of water and in DMC calculations of the ground states of water, CH5 +, and its deuterated analogues. For both of these molecules, comparisons are made to the results obtained using potentials that were fit to much larger sets of electronic energies than were required to train the MOB-ML models. The NN+(MOB-ML) approach is also used to obtain a potential surface for C2H5 +, which is a carbocation with a nonclassical equilibrium structure for which there is currently no available potential surface. This potential is used to explore the CH stretching vibrations, focusing on those of the bridging hydrogen atom. For both CH5 + and C2H5 + the MOB-ML model is trained using geometries that were sampled from an AIMD trajectory, which was run at 350 K. By comparison, the structures sampled in the ground state calculations can have energies that are as much as ten times larger than those used to train the MOB-ML model. For water a higher temperature AIMD trajectory is needed to obtain accurate results due to the smaller thermal energy. A second MOB-ML model for C2H5 + was developed with additional higher energy structures in the training set. The two models are found to provide nearly identical descriptions of the ground state of C2H5 +.
Diffusion Monte Carlo (DMC) is a technique for obtaining the ground-state solution to the vibrational timeindependent Schrödinger equation based on a stochastic sampling of an electronic potential energy surface (PES). To obtain accurate results, one needs a globally-accurate PES. Ideally, the electronic energies used in DMC are calculated at the CCSD(T) level of theory. Recently, neural networks have shown promise to provide accurate post-Hartree corrections to the electronic energy at the cost of a standard Hartree-Fock calculation ab. These neural network surfaces, however, have only been validated using a small sampling of the available configurations of the system. DMC, by design, samples the entire region of the PES that is sampled by the vibrational ground state wave function. Despite the speed of the neural network potential relative to a calculation at a high level of theory such as CCSD(T), the necessity of the evaluation of the Hartree-Fock energy for every one of the thousands of DMC configurations over tens of thousands of time-steps makes this computationally challenging. To this end, we have developed a DMC algorithm to take advantage of massively parallel high performance computing environments. In addition, to further alleviate the computational cost, we have incorporated importance sampling into the algorithm to reduce the overall number of configurations required to adequately sample the PES. Combined, these two optimizations push the calculation back into the realm of computational feasibility. We have benchmarked the calculation on the water monomer and the CH + 5 molecular ion. We plan to extend this work to larger systems, such as large water clusters or a simple Criegee intermediate, where running the DMC with traditional electronic structure would be intractable.
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