Abstract
We present an extension of the polarizable quantum mechanical (QM)/AMOEBA approach to enhanced sampling techniques. This is achieved by connecting the enhanced sampling PLUMED library to the machinery based on the interface of Gaussian and Tinker to perform QM/AMOEBA molecular dynamics. As an application, we study the excited state intramolecular proton transfer of 3-hydroxyflavone in two solvents: methanol and methylcyclohexane. By using a combination of molecular dynamics and umbrella sampling, we find an ultrafast component of the transfer, which is common to the two solvents, and a much slower component, which is active in the protic solvent only. The mechanisms of the two components are explained in terms of intramolecular vibrational redistribution and intermolecular hydrogen-bonding, respectively. Ground and excited state free energies along an effective reaction coordinate are finally obtained allowing for a detailed analysis of the solvent mediated mechanism.
Highlights
In the last few years, there has been a significant progress in the use of hybrid approaches coupling quantum mechanical (QM) and classical models for describing dynamic processes of molecules in solution and more complex environments.1–3 In most cases, a Molecular Mechanics (MM) force field has been used for the classical part of the system and an electrostatic embedding (EE) scheme for the coupling between the QM and the MM subsystems.4–7EE-QM/MM molecular dynamics (MD) simulations have seen many successful applications in the study of ultrafast and fast processes, especially when combined with inexpensive QM methods such as semiempirical methods.8 their application to dynamic processes extending beyond the ps scale is still a challenge due to the overwhelming computational cost, especially if an ab initio QM method is used
In the last few years, we have developed an interface between Tinker51,52 and Gaussian 1629 packages for performing QM/Atomic Multipole Optimized Energetics for Biomolecular Applications (AMOEBA) MD simulations
We have presented an integration of three different software packages (Tinker, Gaussian, and PLUMED), which allows for extending QM/AMOEBA MD simulations on ground and excited states to relatively slow processes, thanks to the use of enhanced sampling techniques
Summary
In the last few years, there has been a significant progress in the use of hybrid approaches coupling quantum mechanical (QM) and classical models for describing dynamic processes of molecules in solution and more complex environments. In most cases, a Molecular Mechanics (MM) force field has been used for the classical part of the system and an electrostatic embedding (EE) scheme for the coupling between the QM and the MM subsystems.. Due to the EE scheme, it is not possible to account for the environment response to time-dependent (TD) changes in the electronic density of the embedded quantum subsystem The latter limitation can be overcome by using a more sophisticated polarizable embedding, which endows the MM atoms with the possibility to dynamically respond to changes in the QM charge distributions, making these approaches more suited to describe excited state (ES) processes.. While the ultrafast component of the process can be simulated using excited state BO-MD as has already successfully been done using BO-MD combined to either a MM solvent and/or an implicit solvent, the slow component happens on a timescale that would render the cost of the simulations prohibitive This motivates us to couple our multiscale machinery with enhanced sampling techniques in order to achieve the description of slower processes at an accessible computational cost. III B, we present the results of the umbrella sampling (US) simulations, which has been used to characterize the slow component of the process
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