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Published byDarcy Gordon Modified over 9 years ago
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Advanced methods of molecular dynamics 1.Monte Carlo methods 2.Free energy calculations 3.Ab initio molecular dynamics 4.Quantum molecular dynamics II 5.Trajectory analysis
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Time-dependent Schrodinger equation: numerically exact solution 1. Discretization of the wavefunction: - expansion into a basis (orthogonal polynomials) - expansion on a grid (equidistant or non-equidistant), n points, k max = h/ vibrations: harmonic oscillator functions – Hermite polynomials rotations: free rotor functons- spherical harmonics
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ih (t)/ t =H (t) (t+ t) = U (t) = e -iH t/ћ (t) i) 2 nd order differences (SOD) – Taylor expansion U U = 1 - iH t/ћ +... (t+ t) = (t) - i t H (t)/ ћ 2. Rozvoj evolučního operátoru U: Numerically unstable - symmetrization (epansion +/- t ) (t+ t) = (t- t) - 2i t H (t)/ ћ conditionally stable: pro t<h/E max error OF the SOD method: O( t 3 ) SOD: Local (small t), simple, but not very accurate method, applicable also to time-dep. Hamiltonians.
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ii) Split Hamiltonian method (S-O): U e -iT t/ћ. e -iV t/ћ...operators T & V non-commuting! U e -iT t/(2h). e -iV t/(2ћ). e -iV t/(2ћ). e -iT t/(2ћ) or U e -iV t/(2h). e -iT t/(2ћ). e -iT t/(2ћ). e -iV t/(2ћ) (equiv.) Error of S-O method: O( t 3 ), but smaller prefactor than SO S-O: Local (small t), more accurate than SO, applicable also to time-dependent Hamiltonians.
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iii) Chebyshev method: U = a n P n (-iHt/ћ)...expansion into orthogonal polynomials P n = cos [n arccos(x)]...n th Chebyshev polynomial a n = 2 J n [(E max -E min )t/(2ћ)]...Bessel functions error of the Chebyshev method ~ e -N Chebyshev: Global (large step), accurate (exponential convergence), applicable only to time- independent Hamiltonians. iii) Other global methods: Lanczos method orthogonal polynomyials by repeated action od Hamiltonian on the wave function tt’- method auxilliary time variable t’, also for H(t)
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3. Operation of Hamilton H on wave function : H = T + V...operators of kinetic and potential energies In coordinate representation: = (x,t), V = V(x) V(x) (x,t)...local multiplication point-by-point on the grid. T=(-iћ/2m) ..nonlocal Laplace diferential operator (x i,t) = ( (x i+1,t) + (x i-1,t) - 2 (x i,t))/ x 2 semilocal approximation – violates uncertainity relations. Fourier transform (x,t) (k,t), multiplication by k 2 inverse Fourier transform. Fast – using FFT.
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Propagation of a wavepacket in 2D Disociation of C-H bonds in acethylene using a train of intense ultrashort IR pulses: intenzivních ultrakrátkých infračervených pulzů: J. T. Muckerman, Brookhaven National Laboratory
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