# convergence of energy&forces when using periodic decoupling

Csilla Varnai cv... at cam.ac.uk
Thu Dec 3 10:26:10 UTC 2009

```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
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.

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
&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
&END KIND
&KIND O
&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

```