Exact lattice calculations of dispersion coefficients in the presence of external fields and obstacles M. G. Gauthier, G. W. Slater and K. D. Dorfman Eur. Phys. J. E, 15 1 (2004) 71-82 Published online: 01 October 2004 DOI: 10.1140/epje/i2004-10037-9