This paper deals with duality based approaches of field
computations. It presents two methods that allow to obtain upper bounds for
the error on the forces or torques. The first one is based on a double
evaluation of the global coenergy for two values of the position. The second
one resembles the popular Maxwell's tensor method and requires the field
computation only once, but respecting an additional rule. For both methods,
the force or torque are considered in respect of their average value on an
interval of displacement, taken as configuration parameter.