TOMBO Ver.2 Manual

1.5 TDDFT dynamics simulation 11
1.5 TDDFT dynamics simulation
In TOMBO, TDDFT dynamics simulation can be performed by setting “A” in iApp in IN-
PUT.inp. If one wants to excite one electron from HOMO to LUMO +n at the outset, one
may define iExcite = 1 + n in INPUT.inp (n can be 0, 1, 2, ...). Default value of iExcite is
0, which means no excitation.
According to the time-dependent density functional theory[13], TOMBO can treat the
time-dependent Kohn–Sham (TDKS) equation
i ∂ ∂t ψ j(r,t) = H q (r,t)ψ j (r,t), (1.32)
by setting “A” in iApp in INPUT.inp Here H q (r,t) is the electronic part of the Hamiltonian.
Combining this time-evolution equation with the Newtonian equation of motion for nuclei
M A ¨R A = − ∂
∂R A
[
E q +V cl] , (1.33)
we can perform semi-classical dynamics simulation within the mean-field-type Ehrenfest
theorem. During one simulation, all dynamical variables change in time along one reaction
path according to the choice of the initial atomic coordinates and initial atomic velocities
(velocities can be written parallel to the right of the coordinates in COODINATES.inp). This
way of simulation is called “on the fly” approach. Here E q is the electronic part of the total
energy, and, together with the Coulomb potential between nuclei V cl , gives the mean-field
potential exerting on nuclei. In these equations, r is the electron coordinate, M A and R A are
the mass and position of the Ath nucleus. This approach is on the fly.
The simulation is basically performed adiabatically, i.e., the atomic motion is assumed
slow enough. However, non-adiabatic simulation is possible by considering the coupling to
the atomic velocity, when the input parameter nonadiabatic = 1 is set in INPUT.inp. Below
only the algorithm of adiabatic simulation is explained, but one may refer to Ref. [14] for
non-adiabatic simulation.
It is necessary to know the exchange-correlation potential to solve Eq.(1.32) step by step
by means of the TDDFT. TOMBO uses a simple LDA exchange-correlation functional that
is local for both space and time. This approximation is called “adiabatic LDA”. In order to
integrate Eq.(1.32) step by step accurately, we use the spectral method[15], in which wave
packet ψ j (r,t) at each instantaneous time is expanded in terms of eigenfunctions ϕ k (r,t)
(eigenvalues are ε k (t))
H q (r,t)ϕ k (r,t) = ε k (t)ϕ k (r,t). (1.34)