资源描述
Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Reactive Empirical Force Fields,Jason Quenneville,jasonqlanl.gov,X-1:Solid Mechanics,EOS and Materials Properties,Applied Physics Division,Los Alamos National Laboratory,Timothy C.Germann,Los Alamos,Alejandro Strachan,Purdue,Adri C.T.van Duin,Caltech,William A.Goddard III,Caltech,Alexei A.Stuchebrukhov,UC Davis,2019 Summer School on Computational Materials Science,July 31-August 11,2019 University of Illinois,Reactive Empirical Force Field,Motivation,Empirical force fields are used in biology,chemistry,physics and materials science to calculate the potential energy surface and atomic forces.,Most,like CHARMM and AMBER,assume the same atomic connectivity(molecular composition)throughout simulation.,No Chemistry!,Straightforward solution:,ab initio,or QM/MM (up to 300 atoms for QM system),For materials simulation,we may want 10s of 1000s to millions of atoms and as much as a nanosecond of simulation time.,Need a more efficient method!,MotivationEmpirical force fiel,Empirical Force Fields,Empirical force fields contain potential energy functions for each atomic interaction in a molecular system.,Bond Stretch:,Bond Bending:,Bond Torsion:,Parameters can be taken from experiment(,e.g.,vibrational spectroscopy)or from,ab initio,quantum chemistry calculations.,Empirical Force FieldsEmpirica,Non-Bonded potentials give the intermolecular interactions:,Coulomb:,van der Waals:,Parameters obtained through ab initio quantum chemistry and liquid simulations.,e.g.OPLS(,optimized potentials for liquid simulations,W.L.Jorgensen and J.Tirado-Rives,J.Am.Chem.Soc.,110,1657(1988).,),Empirical Force Fields,Non-Bonded potentials give the,Empirical Valence Bond(EVB),EVB attempts to combine empirical potential energy functions with valence bond ideas to describe chemical reactions efficiently and accurately.,EVB Applications,Proton transport in aqueous acid,(,CPL,284,71(98);,JPCB,102,5547(98),Aqueous acid-base reactions,(,JPCA,105,2814(01),Enzyme catalysis (Warshel),Nucleophilic substitution reactions,Good Introduction:,Computer Modeling of Chemical Reactions in Enzymes and Solutions,A.Warshel Wiley-Interscience(02/01/2019),Empirical Valence Bond(EVB)EV,EVB:introduction,EVB starts with a N,N potential energy matrix:,N,diabatic states(diagonal),N,(,N,-1)couplings(off-diagonal),Each diabatic state looks like a configuration in a standard non-reactive force field.,Off-diagonal coupling elements:interaction between each diabatic state and the N-1 remaining states.,Diagonalize V,adiabatic states.The minimal value is the ground state.,coupling term,adiabatic ground state,diabatic states,EVB:introductionEVB starts wi,Calculation of Forces,Diagonalizing the,N,N,EVB matrix yields the ground state as a linear combination of diabatic states.,If,a,n,is the set of corresponding coefficients,the forces can be calculated using the Hellman-Feynman theorem:,Calculation of ForcesDiagonali,EVB:diagonal matrix elements,Because we need to treat bond breaking and formation,the Bond Stretch should be anharmonic:,Bending and Torsional potentials can be as before:,System-environment interactions treated with standard non-bonding potentials:,EVB:diagonal matrix elementsB,Interaction between EVB States,System-system non-bonding interactions more complicated due to the potential for chemical reaction.,A functional form more flexible than Coulomb+Lennard-Jones is required.,The intermolecular interactions(part of the diagonal element)and the coupling terms(off-diagonal)must be parametrized together in order to describe the reaction correctly.,In the activated complex,the favorable interaction between the two states is controlled by the intermolecular interaction.It is normally written in terms of the distance between the two reactant centers.,The reaction barrier is controlled by coupling term.This term is generally a function of the reaction coordinate.,Interaction between EVB States,1.2,1.2,0.97,0.97,0.97,0.97,116,118,116,118,109,114,1.22,1.22,0.96,0.96,103,103,101,Optimized geometries of the H,2,OHOH,2,+,(left)and HOHOH,(right)complexes,obtained from first principles(MP2/aug-cc-pVTZ).,Application:Proton Transfer in Water,1.2 1.2 0.97 0.97 0.97 0.,H,H,O,O,H,H,H,H,H,O,O,H,H,H,H,H,O,O,H,H,H,Diabatic States:,Adiabatic State:,EVB of H,3,O,+,H,2,O Proton Transfer,HHOOHHHHHOOHHHHHOOHHHDiabatic,H,3,O,+,H,2,O Proton Transfer:,Diagonal Elements,H3O+H2O Proton Transfer:Diag,H,3,O,+,H,2,O Proton Transfer:,Coupling Elements,H,H,O,O,H,H,H,r,H3O+H2O Proton Transfer:Coup,EVB vs Ab Initio for H,3,O,+,/H,2,O,2.4,2.6,2.8,EVB,MP2/aug-cc-pVTZ,EVB vs Ab Initio for H3O+/H2O2,EVB Summary,Very good for systems with small number of possible reactions,Reaction barriers are treated explicitly,Offers an,empirical,description of chemical reactions,Gives mixing of diabatic states during reaction,Can be difficult to parametrize intermolecular potentials and couplings,Limitatio
展开阅读全文