The authors use local density functional theory to calculate the structure and vibrational properties of small molecules containing phosphorus and selenium atoms in varying concentrations in an attempt to shed light on the composition of zero-dimensional P-Se glasses. They examine the structure and modes of P₄Se_n n=0, 1, 2, 3, 4, 5, 6, 10, P₈Se₈ and P₁₀Se₆. Their results enable them to exclude some molecules from being present. For the Se-rich glasses they find that the singly bonded molecules P₄Se_n, n=3, 4, 5, 6 are stable and are likely to be present. For the P-rich glasses P₄Se₃, P₄Se₂ and possibly P₁₀Se₆ are present.