title: Multilevel X-Pol: a fragment-based method with mixed quantum mechanical representations of different fragments.
abstract: The explicit polarization (X-Pol) method is a fragment-based quantum mechanical model, in which a macromolecular system or other large or complex system in solution is partitioned into monomeric fragments. The present study extends the original X-Pol method, where all fragments are treated using the same electronic structure theory, to multilevel representations, called multilevel X-Pol, in which different electronic structure methods are used to describe different fragments. The multilevel X-Pol method has been implemented into a locally modified version of Gaussian 09. A key ingredient that is used to couple interfragment electrostatic interactions at different levels of theory is the use of the response density for the post-self-consistent-field energy. (The response density is also called the generalized density.) The method is useful for treating fragments in a small region of the system such as a solute molecule or the substrate and amino acids in the active site of an enzyme with a high-level theory, and the fragments in the rest of the system by a lower-level and computationally more efficient method. Multilevel X-Pol is illustrated here by applications to hydrogen bonding complexes in which one fragment is treated with the hybrid M06 density functional, Møller-Plesset perturbation theory, or coupled cluster theory, and the other fragments are treated by Hartree-Fock theory or the B3LYP or M06 hybrid density functionals.
authors.affiliation: Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: Yingjie
authors.initials: Y
authors.lastname: Wang
authors.forename: Carlos P
authors.initials: CP
authors.lastname: Sosa
authors.forename: Alessandro
authors.initials: A
authors.lastname: Cembran
authors.forename: Donald G
authors.initials: DG
authors.lastname: Truhlar
authors.forename: Jiali
authors.initials: J
authors.lastname: Gao
mesh_terms: D019342:Acetic Acid / Q000737:chemistry*; D000596:Amino Acids / Q000737:chemistry; D006860:Hydrogen Bonding; D011789:Quantum Theory; D014867:Water / Q000737:chemistry*
chemical_list: D000596:Amino Acids; D014867:Water; D019342:Acetic Acid