Sorry for the late reply.
I have read the manual for the command "compute stress/atom command" , and I have several questions about the formula to calculate the stress tensor.
If I take x for a and b,the second term for the pair interaction in the formula is 1/2Σ(R1x*F1x+R2x*F2x).The pair interaction is the interactions between two atoms,so only R1x*F1x or R2x*F2x is supposed to appear, but if one image of the atom, and only one image of the atom is taken into account, then the second term which contains both R1x*F1x and R2x*F2x is correct. If the image of the atom is taken into account, for the third term for angled atoms, it is not correct. The reason is that there are two atoms which are bonded with atom I, thus an angle is formed. If the image of the two bonded atoms are taken into account, there should be four terms (two plus two) instead of three,but there are only three terms in the manual for the angle.
Could you please kindly explain why the manual is written so?