Nitrogen is a common structural element in the acceptor moieties of organic donor-acceptor nonlinear optical (NLO) chromophores. In order to assess the effect of substitution with heavier group 15 elements, computational studies of the nonlinear optical properties of betaine derivatives, (CH(3))(3)X(+)CH(2)CO(2)(-) (X = N, P, As, Sb, Bi), have been performed. First hyperpolarisabilities, β(HRS)...