  We propose a new simple computational model to estimate adsorption energies
of atoms and molecules to low-temperature amorphous water ice, and we present
the adsorption energies of carbon (3P), nitrogen (4S), and oxygen (3P) atoms
based on quantum chemistry calculations. The adsorption energies were estimated
to be 14100 +- 420 K for carbon, 400 +- 30 K for nitrogen, and 1440 +-160 K for
oxygen. The adsorption energy of oxygen is well consistent with experimentally
reported value. We found that the binding of a nitrogen atom is purely
physisorption, while that of a carbon atom is chemisorption in which a chemical
bond to an O atom of a water molecule is formed. That of an oxygen atom has a
dual character both physisorption and chemisorption. The chemisorption of
atomic carbon also implies a possibility of further chemical reactions to
produce molecules bearing a C-O bond, while it may hinder the formation of
methane on water ice via sequential hydrogenation of carbon atoms. These would
be of a large impact to the chemical evolution of carbon species in
interstellar environments. We also investigated effects of the newly calculated
adsorption energies onto chemical compositions of cold dense molecular clouds
with the aid of gas-ice astrochemical simulations. We found that abundances of
major nitrogen-bearing molecules, such as N2 and NH3, are significantly altered
by applying the calculated adsorption energy, because nitrogen atoms can
thermally diffuse on surfaces even at 10 K.
