convergence of energy&forces when using periodic decoupling

[Csilla Varnai]

Hi,

I'm trying to get the energy and forces to converge in a periodic QM/ 
MM simulation of water when using the periodic decoupling/recoupling  
scheme.

First of all, I've played around with the *bloechl*inp test files in  
the QS directories to see how to set the parameters to get the best  
accuracy. I monitored the energy and the forces when changing the  
density cutoff (150..500 Ry), the number of grids applied (1..7), the  
Ewald precision(1E-3..1E-12) and the RCUT of the MULTIPOLE section,  
and the quantum cell size(3..13A) (please see below an example input  
file I used).
Blochl suggests 5-7 A separation between periodic images is usually  
enough for the wavefunction separation, when the scheme works, and  
indeed the energy and forces converge beyond about 5 A separation  
(cutoff: 280 Ry, Ewald precision: 1E-8, Rcut: 3A), but the fluctuation  
in the forces is 5E-3 H/b in case of the oxygen (better for H). Also,  
increasing the density cutoff beyond 280 Ry has a 1E-3 H/b fluctuation  
on the oxygen force. Changing the number of grids used may cause 2E-3  
H/b difference in the forces (is this because I didn't reach  
convergence wrt. other parameters or due to inaccuracies in  
interpolation between grids?). The energy fluctuation beyond  
convergence in each case is less than 0.2 mH.

When I go to a system with 1 QM water molecule in a box of 1221 MM  
waters, and I use the decoupling + recoupling scheme (QMMM%PERIODIC  
section) expectedly the energy fluctuation increases up to 0.5 mH, the  
amplitude of the force fluctuation up to 10E-3 H/b (~0.5eV/A), which  
is not acceptable for my calculations.

I would greatly appreciate if anyone could give me suggestions on how  
to improve the force convergence.

Thank you in advance,
Csilla


Here's an example input:

&FORCE_EVAL
     METHOD Quickstep
     &DFT
       BASIS_SET_FILE_NAME ../BASIS_SET
       POTENTIAL_FILE_NAME ../POTENTIAL
       &MGRID
         CUTOFF 280
       &END MGRID
       &POISSON
         PERIODIC NONE
         POISSON_SOLVER MULTIPOLE
         &MULTIPOLE
           EWALD_PRECISION 0.00000001
           RCUT 3.0
           ANALYTICAL_GTERM
         &END MULTIPOLE
       &END POISSON
       &QS
         EPS_DEFAULT 1.0E-12
       &END QS
       &SCF
         EPS_SCF 1.0E-6
         SCF_GUESS ATOMIC
       &END SCF
       &XC
         &XC_FUNCTIONAL Pade
         &END XC_FUNCTIONAL
       &END XC
     &END DFT
     &SUBSYS
       &CELL
         ABC 10.0 10.0 10.0
       &END CELL
       &COORD
       O   4.000000    4.000000    4.000000
       H   4.000000    3.250000    4.520000
       H   4.000000    4.750000    4.520000
       &END COORD
       &TOPOLOGY
         &CENTER_COORDINATES
         &END CENTER_COORDINATES
       &END TOPOLOGY
       &KIND H
          BASIS_SET DZVP-GTH-PADE
          POTENTIAL GTH-PADE-q1
       &END KIND
       &KIND O
         BASIS_SET DZVP-GTH-PADE
         POTENTIAL GTH-PADE-q6
       &END KIND
     &END SUBSYS
   &END FORCE_EVAL
   &GLOBAL
     RUN_TYPE MD
     PROJECT H2O_cell10_rcut3_prec8_cutoff280
     PRINT_LEVEL MEDIUM
   &END GLOBAL
   &MOTION
     &PRINT
       &FORCES
         FORMAT XMOL
       &END FORCES
     &END PRINT
     &MD
       ENSEMBLE NVE
       STEPS 1
       TIMESTEP 0.1
       TEMPERATURE 300.0
     &END MD
- &END MOTION