  We investigate the influence of an inter-chain coupling on the spiral ground
state correlations of a spin-1/2 Heisenberg model consisting of a
two-dimensional array of coupled chains with nearest (J1) and frustrating
next-nearest neighbor (J2) in-chain exchange couplings. Using the
coupled-cluster method we calculate the transition point between the
commensurate and the incommensurate (spiral) ground state as well as the
quantum pitch angle of the spiral ground state. In addition, we provide a
simple empirical formula which describes the relation between the quantum pitch
angle and the frustration parameter J2/J1.
