Molecular Mechanics: The Ab Initio Foundation

Transcription

1 Molecular Mechanics: The Ab Initio Foundation Ju Li GEM4 Summer School 2006 Cell and Molecular Mechanics in BioMedicine August 7 18, 2006, MIT, Cambridge, MA, USA

2 2 Outline Why are electrons quantum? Born-Oppenheimer approximation and the energy surface Hartree-Fock and density functional theory Interatomic potentials

3 The electron problem: basic facts Electrons {x i }, i=1..n: m e = kg Nuclei {X I }, I=1..N: M I A I M p, M p 2000m e They interact electrostatically as V ii = 2 Ze I x X i I I is a carbon ion: A = 12, Z = 6 I I 3

4 Why electrons must be considered quantum, while ions are often considered classical, objects? h de Broglie wavelength: λ = p h = J s: Planck's constant p : momentum p λ Electrons and ions share the same energy scale: 2 2 PI pi ~ 13.6 ev ~, energy of C-C bond 3.5 ev 2M 2m I e 4

5 5 Plugging in the numbers, we get de Broglie wavelength of electron: 3.3 Å hydrogen: 0.08 Å carbon: 0.02 Å Note these are quantum, not thermal, fluctuations electron s wavelength permeates through several structural units (bonds) and so must be treated quantum mechanically carbon s wavelength is well-localized hydrogen is a borderline case.

6 Born-Oppenheimer approximation The electrons minimize their quantum mechanical energy as if the ions are immobile; the resulting total energy Max Born (electrons + ions) is V BO ({X I }), the Born-Oppenheimer energy surface, aka energy landscape, interatomic potential. J. Robert Oppenheimer B-O approximation is also called the adiabatic approximation. The idea is that electrons move so much faster than the ions, that they are at their ground state Ψ G ({x i }) for a given ionic configuration {X I }. Addendum: The ions move classically on the BO energy surface according to Newton s 2 nd law: M I X I V = ({ X I}) X BO I 6

7 BO energy surface aka energy landscape 3N+1 space F I V ({ X I}) X BO I X 2 ionic d.o.f. ionic d.o.f. X 1 7

8 BO approximation breaks down when 1. The molecule is optically excited (electronic excited states) 2. During diabatic electron transfer process (Marcus theory) Ψ B Ψ A w A = VAB B 2 π λkt B e 1 ( EA EB+ λ ) k T 4λ B 2 {X I } Addendum breaks down when 1. For light-mass ions like hydrogen, or at low temperature k B T<<ħω D (Then, even the ions need to be treated quantum mechanically.) 8

9 The quantum mechanical life of electrons H Ψ ( x, x,..., x ) = V Ψ ( x, x,..., x ) G 1 2 N BO G 1 2 N H In the non-relativistic limit: n i e ZIe = + 2m 2 x x x X i= 1 e i j i j i, I i I + 2 ZZe I J 2 X X I J I J If one attempts to get at V BO by solving the above equation under rational and non-material-specific approximations, without using any experimental input, the result should then depend on and only depend on numerical values of ħ, m e, e, {Z I }. This is called ab initio calculation. 9

10 The problem of information explosion To store a single-variable function f(x), 0<x<1: use 10 spline points, each spline data (in double precision) is 8 bytes: 80 bytes We have 20 electrons in a box, 0<x<1, 0<y<1, 0<z<1 Ψ G (x 1, x 2,, x 20 ) = Ψ G ( x 1, y 1, z 1, x 2, y 2, z 2,, x 20, y 20, z 20 ) A 60-dimensional function: needs spline data points total storage required: bytes A CD can store bytes: needs CD s Say each CD is one gram, gram Mass of the sun: gram 10

11 There are some symmetry relations: the electrons are indistinguishable Fermions: Ψ ( x, x,..., x,..., x,..., x ) = Ψ( x, x,..., x,..., x,..., x ) 1 2 i j n 1 2 j i n but that does not solve the explosion fundamentally Nobel Prize in Chemistry John A. Pople Walter Kohn quantum chemistry density-functional theory (DFT) Progress in the last 50 years has been tremendous. Significant number of researchers most cited chemists, most cited physicists. 11

12 from Thom H. Dunning, Jr. 12

13 MP2: Møller-Plesset perturbation theory 2 nd order CCSD: Coupled cluster with single- and double-excitations CCSD(T): plus triple excitations calculated by perturbation theory 13

14 Hartree-Fock Theory Assume Ψ(x 1, x 2,, x n ) can be well-approximated by a single Slater determinant: S = 1 n! ψ ( x ) ψ ( x ) ψ ( x ) n 1 ψ ( x ) ψ ( x ) ψ ( x ) n 2 ψ ( x ) ψ ( x ) ψ ( x ) 1 n 2 n n n 3n-dimensional function n 3-dimensional functions: ( x), ( x),..., ( x) ψ1 ψ2 ψ n If n=20, = 160 kilobytes bytes 160 kilobytes compression 14

15 Each trial wave function also contains spin information: ψ ( x) = ψ( x) or ψ( x) : just 1-bit extra Contribution to total energy due to ψ and = E Hartree [ ψ, ψ ] - E [ ψ, ψ ] i j Exchange i j i ψ j [ ψ, ψ ] x x 2 EHartree i j e d d ρ ( ) j ( ) i x ρ x i i j j ( ψ ( x) ψ ( x))( ψ ( x ) ψ ( x )) x x x x 2 EExchange[ ψi, ψ j] e d d ( ψ ( x) ψ ( x))( ψ ( x ) ψ ( x )) i j i j x x 15

16 E [, ] > 0 Hartree ψ i ψ j does not care if ψ i, ψ j have the same spin E Exchange 0 if ψ i, ψ j have different spin [ ψ i, ψ j] = >0 if ψ i, ψ j have same spin An occupied wavefunction does not see itself in Hartree-Fock theory: E Hartree E Hartree [ ψ, ψ ] = E [ ψ, ψ ] i i Exchange i i [ ψ, ψ ]- E [ ψ, ψ ]=0 i i Exchange i i No self-interaction Exchange interaction stabilizes occupation of same-spin wavefunction with large spatial overlap. 16

17 Due to the structure of the Slater determinant, two electrons of the same spin automatically avoid each other, creating so-called exchange-hole in their pair-correlation function. Hartree-Fock is beautiful. But in reality: Ψ G = as 1 1+ as as each S µ is a Slater determinant By optimizing the coefficients a 1, a 2, a 3, and also the Slater determinants, one can further reduce the energy beyond the best single-determinant (Hartree-Fock) energy. This energy reduction is called correlation energy. The brute-force way of doing above is called configuration interaction (CI). 17

18 Both exchange and correlation energies stablize a manyelectron system beyond naïve Coulomb interactions. Exchange energy tends to be larger in magnitude (10 ) than correlation energy. Full CI is formally exact, but it has very bad scaling, something like O(n 10 ) The present gold standard, CCSD(T), maintains most of the accuracy and has better scaling. But it is still expensive, something like O(n 7 ), so still limited to small molecules, say ~20 atoms. In this sense, density functional theory (DFT) is a poor man s way of taking account of both exchange and correlation. It tends to be cheaper than even Hartree-Fock. 18

19 DFT is the technology that underlies most of the condensedmatter research, as well as a very significant part of biomolecular modeling. Hohenberg and Kohn, Phys. Rev. 136 (1964) B864: V BO ({X I }) = V BO [ρ(x)] ρ(x): single-electron density at ground state v(x): ion-electron (external) potential Ψ G (x 1, x 2,, x n ) So from informatics point of view, instead of treating n 3-dimensional functions, one formally only needs to treat one 3-dimensional function bytes 8 kbyte 19

20 Kohn and Sham, Phys. Rev. 140 (1965) A1133: It is still exact, and physically expedient, to decompose V BO [ρ(x)] = V Independent [ρ(x)] + V e-e Hartree [ρ(x)] + V Exchange-correlation [ρ(x)] where V Independent [ρ(x)] is the energy of a fictitious, independent-electrons system having the same ρ(x), where V n 2 2 independent[ ρ( x)] = dxψi ( x) ψi( x) i= 1 2me 2 2 Ze I ρi( x) ZZe I J dx +, x X 2 X X ii, I I J I J n n i i i i= 1 i= 1 ρ( x) ψ ( x) ψ ( x) ρ ( x). kinetic energy: difficult to express well (either in fictitious- or real-electrons system) as a local functional of ρ(x). 20

21 ψ1 x ψ2 x ψ n x 60 functions bytes 160 kbyte. ( ), ( ),..., ( ) are called Kohn-Sham wave In appearance, they look similar to Hartree-Fock trial wave functions. But their interpretations are shadowy, and it would be inappropriate to call DFT a "single configuration" (single determinant) method. Local density approximation (LDA): V Exchange-correlation[ ρ( x)] dxρ( x) vxc( ρ( x)) Perdew and Zunger parameterized v XC (ρ) using the Quantum Monte Carlo data by Ceperley and Alder (1980) for homogeneous electron gas. 21

22 By definition, LDA works well when the electron density is nearly uniform, for instance inside a simple metal. But when the electron density varies violently, for instance in gas-phase molecules, LDA could fail. Various attempts of Generalized Gradient Approximation (GGA), such as PW91 and PBE96, improve results somewhat in condensed phases such as water, but serious problems remain for molecules. An important reason is LDA/GGA sometimes underestimates exchange energy self-interaction. Comes in hybrid functionals (Becke, J. Chem. Phys. 98 (1993) 5648): The DFT exchange energy is mixed with Hartree-Fock exact exchange (nonlocal). 22

23 Hybrid functionals such as B3LYP work well for biomolecular systems, but because of the mixing parameter, many people do not consider them true ab initio methods. Local orbital basis: Gaussian, NWChem, GAMESS, DMol, SIESTA, 23 DFT typically treat hundreds of atoms with good basis. Currently very active developments: orbital-dependent density functionals, LDA+U, self-interaction correction (SIC), time-dependent DFT (TDDFT), Planewave codes: VASP, PWSCF, CPMD, ABINIT, DACAPO, CASTEP

24 Semi-Empirical Electronic-Structure Methods (Hückel Theory, Linear Combination of Atomic Orbitals, Molecular Orbital Theory, Empirical Valence Bond, Tight-Binding, ) Typically treat thousands of atoms with minimal basis. Experimental or ab initio information used to fit intrinsic electronic quantities: orbital overlap, hopping integral, etc. For a given ion configuration {X I }: an electronic Hamiltonian is first assembled, and then diagonalized, in usage. Parameter sets: AM1, PM3, PM5 codes: MOPAC Also implemented in: Gaussian, GAMESS, CAChe 24

25 Interatomic Potential / Force Field Direct parameterization of V BO ({X I }) without touching the electronic degrees of freedom, in usage. I = 1..N: typically N= l θ ω Leach, Molecular modelling: principles and applications (Prentice-Hall, New York, 2001). 25

26 A typical Force Field for macromolecules looks like: BO bond stretch: pair bending: triplet k h V ({ X }) = ( l l ) + ( θ θ ) I 0 2 I 0 2 I I I I I bonds 2 angles 2 covalent dihedral angles torsion: quartet gi + (1 + cos( m( ωi ω ))) 2 non-bond dispersive interaction: pair 12 6 σ IJ σ IJ + 4ε IJ I> J rij r IJ 0 2 I covalent & delocalized π bonding long-range correlation + 2 qqe I J r non-bond Coulomb interaction: pair I> J IJ ionic 26

27 Dispersive / van der Waals interaction: 12 6 σ σ V() r 4ε = r r V/ε -r -6 : dipole-dipole polarization fluctuations r/σ 27

28 Treatment of Long-Range Electrostatic Interactions: Ewald sum: Decompose 1/r into Long-range smooth + Short-range sharp contributions Long-range smooth part is summed in reciprocal (k-) space Short-range sharp part is summed in real space Modern techniques such as Fast Multipole, Particle Mesh Ewald methods further enhance the efficiency to nearly O(N). Further Readings: Leach, Molecular modelling: principles and applications (Prentice-Hall, New York, 2001). Jensen, Introduction to computational chemistry (Wiley, New York, 1999). Schlick, Molecular Modeling and Simulation (Springer-Verlag, Berlin, 2002). 28