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. The calculated optimized geometry is in a good agreement with experiment. The population analysis showed that the crystal has an ionic-covalent chemical bonding. 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.