Thermodynamic perturbation theory was applied to find the second-order
term to the free energy approximation of the semiinfinite rectilinear
atomic chain and the fee crystal with surface, whose interatomic pote
ntial was described by the Morse function. The system of harmonic osci
llators was chosen as the system of origin. The second order term was
shown to be of the same order of magnitude as the first, and there is
a temperature at which the chain, the square lattice and the fee latti
ce lose their mechanical stability. There is a simple way to End the H
elmholtz free energy of the system in this approximation. The expressi
ons obtained were used to calculate the lattice constant, the mean-squ
are displacement of an atom (MSD) and other parameters of the surface
and the bulk of a monocrystal, and also the temperature dependencies o
f these parameters. As temperature increases, surface relaxation (the
displacement of the top layer) increases, while the anisotropy of the
MSD of surface atoms changes slightly. The data on anisotropy of the M
SD of surface atoms are in good agreement with available experimental
results.