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[prasen@mri.ernet.in]

Dear Abinitioners:

I am doing a calculation on Si(100) surface. First, I am trying to get
the p(2x1) reconstruction, to be used for further calculations. I have a
couple of questions about convergence of this system.

I am starting from atomic positions slightly disturbed from an ideal
surface--alternate top layer atoms moved slightly (~0.1 A) towards the 
possible
dimerization direction. BTW, I have 5 Si layers 16 atoms per layer,
bottom layer saturated by H, and 9 A vacuum. I use ionmov=2 (BFGS)
scheme for atomic relaxation.

1. For the first 4 Bryoden steps, SCF converges within 4-5 cycles
(nstep).  However, from the 5th Broyden step onwards, SCF does not converge
even within 60 cycles. The energy changes are quite large and there
seems no tendency of convergence. Any help as to what might cause this?
Unless SCF converges, probably the forces would be wrong.

May be related to this...
2. From the 3rd Broyden step onwards, some atoms are moved
opposite to the direction of forces on them. Is this possible in BFGS
scheme?

3. Is it good to use ionmov=2 when there are a large no. of atoms (~96)
to be relaxed, or should I use some other scheme?

Values of some parameters that may be related to convergence:
diecut = 2.3        diemac = 12.0   diemix = 0.5   iprcel = 45  iscf = 5

Getting the p(2x1) reconstruction should not be too difficult I guess.
Am I missing somethiing? Any help is appreciated, and thanks in advance.

Prasenjit Sen.
Harish-Chandra Research Inst.
Jhnsi, Allahabad 211019
INDIA.