  Several full-dimensional potential energy surfaces (PESs) are reported for
vibrating CO adsorbates at two coverages on a rigid NaCl(100) surface based on
first principles calculations. These PESs reveal a rather flat energy landscape
for physisorption of vibrationless CO on NaCl(100), evidenced by various C-down
adsorption patterns within a small energy range. Agreement with available
experimental results is satisfactory, although quantitative differences exist.
These PESs are used to explore isomerization pathways between the C-down and
higher energy O-down configurations, which reveal a significant isomerization
barrier. As CO vibration is excited, however, the energy order of the two
isomer changes, which helps to explain the experimental observed flipping of
vibrationally excited CO adsorbates.
