The cross product of two bond vectors yields a directional vector, this is used to determine the alpha face direction. NAMOT2 uses this user provided information to orient the unit and bases after the diagonalization of the moment of inertia matrix. The N1-C2 and C2-N3 vectors are used to calculate the cross product. For purines a positive cross product is defined as Alpha face up, for pyrimidines a negative cross product is defined as Alpha face up.