Recent direct mechanical measurements of atomic force microscopy showed tha
t the force between the silicon tip and the silicon sample is long range in
the attractive region and its magnitude at maximum is relatively smaller.
These features disagree with previous theoretical predictions based on the
ab initio calculations. We investigated the nature of forces between a sili
con tip and the silicon (111)-(2X1) surface by performing first-principles
pseudopotential and classical molecular dynamics calculations and by calcul
ating the van der Waals interaction. The first two methods give forces that
are short range in nature. Fair agreement between the experiment and theor
y is obtained when the van der Waals interaction is included. The effect of
the tip induced deformation is analyzed. [S0163-1829(99)04504-X].