Calculation of matrix elements José M. Soler Universidad Autónoma de Madrid
Schrödinger equation H i (r) = E i i (r) i (r) = c i (r) (H - E i S )c i = 0 H = S =
Kohn-Sham hamiltonian H = T + V PS + V H (r) + V xc (r) T = -(1/2) 2 V PS = V ion (r) + V nl V ion (r) - Z val / r Local pseudopotentiual V nl = | > < | Kleinman-Bylander V H (r) = dr’ (r’) / |r-r’| Hartree potential V xc (r) = v xc ( (r)) Exchange & correlation
Long-range potentials H = T + V ion (r) + V nl + V H (r) + V xc (r) Long range V H (r) = V H [ SCF (r)] - V H [ atoms (r)] V na (r) = V ion (r) + V H [ atoms (r)] Neutral-atom potential H = T + V nl + V na (r) + V H (r) + V xc (r) Two-center integrals Grid integrals
Sparsity KB pseudopotential projector Basis orbitals Non-overlap interactions with 1 and 2 2 with 1,2,3, and 5 3 with 2,3,4, and 5 4 with 3,4 and 5 5 with 2,3,4, and 5 S and H are sparse is not strictly sparse but only a sparse subset is needed
Two-center integrals Convolution theorem
Atomic orbitals
Overlap and kinetic matrix elements K 2 max = 2500 Ry Integrals by special radial-FFT S lm (R) and T lm (R) calculated and tabulated once and for all
Real spherical harmonics
Grid work FFT
Poisson equation 2 V H (r) = - 4 (r) (r) = G G e iGr V H (r) = G V G e iGr V G = - 4 G / G 2 (r) G V G V H (r) FFT
GGA
Egg-box effect Affects more to forces than to energy Grid-cell sampling E x Grid points Orbital/atom
Grid fineness: energy cutoff x k c = / x E cut =h 2 k c 2 /2m e
Grid fineness convergence
Points and subpoints Sparse storage: (i, i ) Points (index and value stored) Subpoints (value only) x E cut
Extended mesh 7 8,12,13 2,4,5 6 14 15,19,20 4,3,1 +1,+5,+6
Forces and stress tensor Analytical: e.g. Calculated only in the last SCF iteration