[Pw_forum] STM code questions

[David Pullman]

Hello,
I’m trying to modify stm.f90 in order to 
implement Julian Chen’s “derivative rule” for 
simulating STM images.  For example, if the 
active orbital of the tip is pz, the contribution 
of each surface state to the STM signal is 
proportional to the square of the z-derivative 
|dpsi/dz|^2, where psi is the surface wavefunction.

Before I get too far, I have two questions about the code:

1)  The wavefunction (psic) is stored in a 1-D 
array.  After psic has been transformed to real 
space, does the index run over a, then b, then c 
(a,b,c are the lattice vectors)?  That 
is:  a1,b1,c1
an,b1,c1; a1,b2,c1
an,b2,c1; 
a1,bn,c1
an,bn,c1;  a1,b1,c2
an,b1,c2;


2)  There’s a comment at the beginning of 
stm.f90, “It may not properly work if the slab 
has two symmetric surfaces.”  Why would symmetric surfaces present a problem?

Thanks,

David Pullman
Department of Chemistry and Biochemistry
San Diego State University
San Diego, CA  92182-1030