Abstract
In this work, we present a novel force-based scheme to perform hybrid quantum mechanics/molecular mechanics (QM/MM) computations. The proposed scheme becomes especially relevant for the simulation of host-guest molecular systems, where the description of the explicit electronic interactions between a guest molecule and a classically described host is of key importance. To illustrate its advantages, we utilize the presented scheme in the geometry optimization of a technologically important host-guest molecular system: a pentacene-doped p-terphenyl crystal, a core component of a room-temperature MASER device. We show that, as opposed to the simpler and widely used hybrid scheme ONIOM, our Quantum-Coupling QM/MM scheme was able to reproduce explicit interactions in the minimum energy configuration for the host-guest complex. We also show that, as a result of these explicit interactions, the host-guest complex exhibits an oriented net electric dipole moment that is responsible for red-shifting the energy of the first singlet-singlet electronic excitation of pentacene.
Highlights
Host–guest systems are important in a wide variety of research fields
Modeling host–guest systems with a purely quantum mechanical formalism is computationally very expensive, and the analysis of the electronic structure is often limited to geometries that are not in equilibrium according to the method selected to perform the quantum mechanical computations; these configurations may come from experimental determinations or geometry optimizations with different methods
While better quality descriptions may be achieved by widening the usage of the quantum formalism, it is important to bear in mind that every quantum mechanics/molecular mechanics (QM/molecular mechanics (MM)) scheme imposes some level of trade-off between the required accuracy and a feasible computational speed
Summary
Host–guest systems are important in a wide variety of research fields. Their computational simulation is relevant in chemistry,[1,2,3] biology,[3,4,5,6,7,8,9,10] and materials science.[3,11,12,13,14,15]. We will employ the standard mechanical coupling hybrid scheme, ONIOM, for the geometry optimization of the system Both schemes can correctly describe the host structure with the same classical force field, we will show that our quantum coupling scheme (QC-QM/MM) is able to produce configurations that exhibit host–guest interactions capable of influencing the generation of photo-induced excitons in the guest molecule. Our hybrid scheme is relevant for the simulation of host–guest molecular systems where a quantum coupling is required to extend the study of the electronic interactions to the boundary between the fragments without sacrificing the quality of the condensed phase description This is achieved by ensuring that the intramolecular forces for all the host molecules are calculated using a single classical force field, able to adequately reproduce the structure of the condensed phase. While better quality descriptions may be achieved by widening the usage of the quantum formalism, it is important to bear in mind that every QM/MM scheme imposes some level of trade-off between the required accuracy and a feasible computational speed
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