Click here for input/output files and main program source code for Example 3
This example illustrates the use of a nonstandard distribution of surface states, applicable to π-bonded chains on the Si(111)2x1 surface, together with the definition of two different types of domains for the chains (with different band gaps). A checker board arrangement of domains is defined, using the the routine IGETAR located in the same file as the Mult3.f main program:
INTEGER FUNCTION IGETAR(X,Y)
COMMON/DOMAINS/DSIZE
C
IXGRID=(INT(ABS(X)/(DSIZE/2.))+1)/2
IYGRID=(INT(ABS(Y)/(DSIZE/2.))+1)/2
IF (MOD(IXGRID,2).EQ.0.AND.MOD(IYGRID,2).EQ.0) THEN
IGETAR=1
ELSE
IGETAR=2
END IF
RETURN
END
with the parameter DSIZE defining the size of the areas provided in the FORT.9 input file. Note that the temperature dependence of the surface state occupation is included (line 40 of the FORT.9 input file is set to 1), so that a few minutes of computation time is required to construct the tables of surface charge density. A large tip-sample separation (10 nm) is also used, in order to evaluate the surface potential distribution without significant effects of the tip itself.
Output to FORT.11 and FORT.12 gives the electrostatic potential energy (column 2) vs. the z-distance through the vacuum and semiconductor (column 1, in FORT.11) or the radial distance along the surface (column 1, FORT.12). The latter cut is taken at a user-specified angle of 0 degrees relative to the x-direction, with the cut actually corresponds to the closest available angle to that which is 2.8 degrees relative to the x-direction (as listed near the bottom of the FORT.16 output file). When plotted, these potentials appear as: