Perils of ad hoc approximations for the activity function of chemically powered colloids M. N. Popescu, W. E. Uspal, M. Tasinkevych and S. Dietrich Eur. Phys. J. E, 40 4 (2017) 42 Published online: 11 April 2017 DOI: 10.1140/epje/i2017-11529-1