The Schmid law, which has been ubiquitously utilized in large-scale continuum computations, asserts that only the shear stress acting in the slip plane in the slip direction controls the plastic deformation. This rule is accurate for fcc metals but it does not apply in bcc metals owing to the spacial spreading of the core of screw dislocations that control their plastic deformation. In this paper we present resulsts of atomic-level simulations of the effect of externally applied stresses on 1/2[1 1 1] screw dislocations in molybdenum. We concentrate on the effect of other components of the applied stress tensor than the Schmid stress in the slip plane ( 1 0 1). These are shear stresses parallel to the Burgers vector in other {1 1 0} planes of the [1 1 1] zone as well as shear stresses perpendicular to the Burgers vector acting in {1 1 0} planes. We thus identify three distinct non-glide shear stresses that affect the glide of 1/2<1 1 1> screw dislocations and formulate single crystal yield criteria that include the effects of these stress components. This forms a basis for multislip yield criteria and flow relations for continuum analyses. Using this approach we demonstrate that the effects of non-glide stresses that originate at the level of individual dislocations have significant influence on plastic yielding of polycrystals.