Low scaling MBE(3)-OSV-MP2 analytical gradient theory and application

Abstract

By utilizing local approximation, sparse tensor representations of the wavefunction can be constructed for electronic structure methods. For example, orbital-specific-virtual (OSV) approximation decreases the complexity of manipulating the wavefunction to calculate observables. Beyond the existing low scaling ab-initio models for predicting single-point energies for large molecules, analytical energy derivatives of the local ab-initio theory are required to handle a wide range of chemistry problems, such as molecular geometries, reaction pathways, thermal and spectroscopic properties, and so on.

We have devised and implemented a method that introduces many-body expansion (MBE) for efficiently computing analytical energy gradients from the second-order Mollet-Plesset perturbation theory with orbital-specific-virtual (OSV-MP2). The third-order many-body expansion of OSV-MP2 amplitudes and density matrices was developed to incorporate orbital-specific clustering and long-range screening techniques, thereby eliminating the need for term-by-term differentiation of the MBE energy bodies. We have further optimized our method by taking advantage of algorithmic sparsity, which allows for the removal of remote fitting integrals and OSV relaxations. Our implementation has been successfully benchmarked on a variety of molecules, exhibiting economic scaling in both linear and quadratic regimes for computing MBE(3)-OSV-MP2 amplitude and gradient equations, respectively. The results show that the accuracy of MBE(3)-OSV-MP2 is comparable to the original OSV-MP2. Additionally, we have integrated MPI-3-based parallelism into the MBE(3)-OSV-MP2 algorithm using shared memory one-sided communication. To obtain better parallel scalability and memory accessibility, we have sorted MBE(3) orbital clusters into independent tasks that are distributed across multiple processes and nodes, where MBE(3)-OSV-MP2 intermediates are accommodated in either global or local memory locations dependent on data sizes.

For an illustrative application, number-adaptive MBE(3)-OSV-MP2/ molecular mechanical Born-Oppenheimer molecular dynamics simulations are conducted for Criegee intermediates (CIs) CH2OO and anti-CH3CHOO, important short-lived intermediates generated from ozonolysis in the troposphere, at the surface of a large water microdroplet. The results obtained from computations indicate a significant transfer of water charge, with each water molecule transferring up to approximately 20\%. This transfer results in the generation of radical pairs of H2O+/H2O- at the air/water interface, which enhances the reactivity of CH2OO and anti-CH3CHOO with water. The strong electrostatic attraction between CI and H2O- at the microdroplet surface promotes the nucleophilic attack on the CI carbonyl by water, which may offset the substituent's steric hindrance and expedite the CI-water reaction. Through statistical analysis of molecular dynamics trajectories, we discovered an intermediate state, CI(H2O-), that remains bound for a comparatively long period at the air/water interface, which is not observable in gaseous CI reactions. This work provides valuable insights into what could potentially modify the oxidizing potency of the troposphere by CIs larger than CH2OO. Moreover, it offers a fresh perspective on the function of interfacial water charge transfer in accelerating chemical reactions on microdroplet surfaces.