VARRLX - abinit @ ウィキ

VARRLX

構造緩和、構造最適化のための変数

dtion

１ステップの時間。単位は(h/2πHa)=0.02418884fs。
デフォルトは100。

ionmove

0=> do not move ions (イオンを動かさない）

1=> move atoms using molecular dynamics with optional viscous damping (friction linearly proportional to velocity). The viscous damping is controlled by the parameter "vis". If actual undamped molecular dynamics is desired, set vis to 0. The implemented algorithm is the generalisation of the Numerov technique (6th order), but is NOT invariant upon time-reversal, so that the energy is not conserved. The value ionmov=6 will usually be preferred, although the algorithm that is implemented is lower-order. The time step is governed by dtion. opcell/=0 is not available

2=> conduct structural optimization using the Broyden-Fletcher-Goldfarb-Shanno minimization (BFGS). This is much more efficient for structural optimization than viscous damping, when there are less than let's say 10 degrees of freedom to optimize.

3=> conduct structural optimization using the Broyden-Fletcher-Goldfarb-Shanno minimization (BFGS), modified to take into account the total energy as well as the gradients (as in usual BFGS).
See the paper by Schlegel, J. Comp. Chem. 3, 214 (1982). Might be better than ionmov=2 for few degrees of freedom (less than 3 or 4)

4=> conjugate gradient algorithm for simultaneous optimization of potential and ionic degrees of freedom. It can be used with iscf=2 and iscf=5 or 6 (WARNING : this is under development, and does not work very well in many cases). optcell/=0 is not available.

5=> Simple relaxation of ionic positions according to (converged) forces. Equivalent to ionmov=1 with zero masses, albeit the relaxation coefficient is not vis, but iprcfc. optcell/=0 is not available.

6=> Molecular dynamics using the Verlet algorithm, see Allen & Tildesley "Computer simulation of liquids" 1987, p 81. Although partly coded, optcell/=0 is not available. The only related parameter is the time step (dtion).

7=> Quenched Molecular dynamics using the Verlet algorithm, and stopping each atom for which the scalar product of velocity and force is negative. Although partly coded, optcell/=0 is not available. The only related parameter is the time step (dtion). The goal is not to produce a realistic dynamics, but to go as fast as possible to the minimum. For this purpose, it is advised to set all the masses to the same value (for example, use the Carbon mass, i.e. set amu to 12 for all type of atoms).

8=> Molecular dynamics with Nose-Hoover thermostat, using the Verlet algorithm. Although partly coded, optcell/=0 is not available. Related parameters : the time step (dtion), the initial temperature (mditemp), the final temperature (mdftemp), and the thermostat mass (noseinert).

9=> Langevin molecular dynamics. Although partly coded, optcell/=0 is not available. Related parameters : the time step (dtion), the initial temperature (mditemp), the final temperature (mdftemp), and the friction coefficient (friction).

12=> Isokinetic ensemble molecular dynamics. The equation of motion of the ions in contact with a thermostat are solved with the algorithm proposed by Zhang [J. Chem. Phys. 106, 6102 (1997)], as worked out by Minary et al [J. Chem. Phys. 188, 2510 (2003)]. The conservation of the kinetic energy is obtained within machine precision, at each step.
Although partly coded, optcell/=0 is not available. Related parameters : the time step (dtion), the initial temperature (mditemp), the final temperature (mdftemp), and the friction coefficient (friction).

13=> Isothermal/isenthalpic ensemble. The equation of motion of the ions in contact with a thermostat and a barostat are solved with the algorithm proposed by Martyna, Tuckermann Tobias and Klein [Mol. Phys., 1996, p. 1117].
optcell/=0 is available. Related parameters : the time step (dtion), the initial temperature (mditemp), the final temperature (mdftemp), the number of thermostats (nnos), and the masses of thermostats (qmass). If optcell=1 or 2, the mass of the barostat (bmass) must be given in addition.

14=> simple molecular dynamics with a symplectic algorithm proposed by S.Blanes and P.C.Moans [called SRKNa14 in Practical symplectic partitioned Runge--Kutta and Runge--Kutta--Nyström methods, Journal of Computational and Applied Mathematics archive, volume 142, issue 2 (May 2002), pages 313 - 330] of the kind first published by H. Yoshida [Construction of higher order symplectic integrators, Physics Letters A, volume 150, number 5 to 7, pages 262 - 268]. This algorithm requires at least 14 evaluation of the forces (actually 15 are done within Abinit) per time step. At this cost it usually gives much better energy conservation than the verlet algorithm (ionmov 6) for a 30 times bigger value of dtion. Notice that the potential energy of the initial atomic configuration is never evaluated using this algorithm. Option optcell/=0 is not available.

mdftemp

MD温度の目標値。単位はK。

mditemp

MD温度の初期値。単位はK。

optcell

optcell=0 : modify nuclear positions, since ionmov=2, but no cell shape and dimension optimisation.
optcell=1 : optimisation of volume only (do not modify rprim, and allow an homogeneous dilatation of the three components of acell)
optcell=2 : full optimization of cell geometry (modify acell and rprim - normalize the vectors of rprim to generate the acell). This is the usual mode for cell shape and volume optimization. It takes into account the symmetry of the system, so that only the effectively relevant degrees of freedom are optimized.
optcell=3 : constant-volume optimization of cell geometry (modify acell and rprim under constraint - normalize the vectors of rprim to generate the acell)
optcell=4,5 or 6 : optimize acell(1), acell(2) or acell(3), respectively (only works if the two other vectors are orthogonal to the optimized one, the latter being along its cartesian axis).
optcell=7,8 or 9 : optimize the cell geometry while keeping the first, second or third vector unchanged (only works if the two other vectors are orthogonal to the one left unchanged, the latter being along its cartesian axis).

strtarget

目標圧力。単位は(Ha/Bohr^3)。セルを最適化する際に必要。
1(Ha/Bohr^3)=29421.033GPa。
テンソル表記もできる。