The phosphorus pentoxide crystal with Pnam symmetry containing four P₂O₅ molecules in the unit cell was calculated by ab initio density-functional theory in the local-density approximation with a plane-wave basis set using the CASTEP code. The calculated optimized geometry is in good agreement with experiment. The population analysis made in terms of Löwdin and Mulliken charges and Wiberg indexes showed that this crystal has a mixed chemical bonding, partly ionic, partly covalent. An Ag atom impurity in the phosphorus pentoxide crystal was investigated within the periodic model. The distortion of the lattice around the impurity was analyzed in terms of PO₄ tetrahedra. The calculations showed that in spite of considerable deformation of the lattice, the Ag impurity does not break the P-O bonding network and changes the P-O bond order only a little. The Ag impurity atom is bound to the crystal with an almost pure ionic bond, the Ag atomic charge being 0.6 (Löwdin charge).