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["David M. Wood" <dmwood@mines.edu>]

Greetings all!

I'm examining the 'bonding' of a (neutral) molecule to a ZnO Zn-terminated 
surface (using a 5 bilayer slab geometry) and, in order not to bias the 
orientation of the molecule, started force relaxation [ionmov=3] (with the 
molecule 'above' the surface) with the bottom atom of the molecule 10 bohr 
above the 'top' surface atom.  To my horror, after 75 Broyden steps all I've 
seen is small (about 0.1 bohr) oscillations about the z value where I started 
the molecule. The molecule has *twisted* slightly in place around its 
original axis.  With ionmov=7 (quenched Verlet molecular dynamics) the same 
thing occurs.  I'm going to die of old age before these calculations 
converge.  In some sense this (not my death) is reassuring  since very 
different approaches to structural relaxation exhibit the same (annoying) 
physics.

I assume that the problem lies in intra-molecular forces far exceeding the 
molecule-surface atom force at this distance, so that the neutral molecule 
experiences only a weak torque due to an (inhomogeneous) electric field.  
Even a rigid rotation of the molecule might then induce a long cascade of 
slight readjustment of Cartesian coordinates that change the total energy 
very little and very very slowly.  I found that optimizing isolated molecules 
(in a large-lattice-constant cubic cell) was extremely easy by comparison.

So, a several obvious questions:

(0) Which mode of IONMOV is most resistant to being trapped in local minima 
of the energy as a function, say, of the molecular orientation?

(1) LARGE MAIN QUESTION: Is there a simple way within ABINIT to freeze the 
RELATIVE coordinates of the atoms in a molecule and allow the "rigid" 
molecule to rotate and move as a block in response to external forces (in my 
case, due to the atoms in the slab).

(2) SMALL AUXILIARY QUESTION: Is there a general protocol for such 
relaxations: e.g., first do a not-so-well converged relaxation starting from 
no more than 'xxx' bohr from the surface, then do a refinement once a 
plausible near-equilibrium geometry has been found?  (Yes, xxx would 
obviously depend on nasty details: I should estimate surface charges and 
hence the non-uniform electric field, the molecule's dipole moment, and hence 
the non-uniform field force and insist this exceed intra-atomic forces.)

I'm using ABINIT 5.8.4 but believe my problems have little to do with ABINIT 
per se.  Key parts of my input file:

occopt 7 
tsmear 0.01 
ionmov 3
toldff 1.d-6 #this is for ELECTRON SCF: 10x bigger than tolmxf:
tolmxf 1.d-5 #somewhat finer than before (sep 15)

Many thanks for any suggestions!

DMW


David M. Wood, Dept. of Physics, Colorado School of Mines, Golden, CO 80401
Phone: (303) 273-3853; Fax: (303) 273-3919