where a ij for a pair ij is defined as a i + a j , while the term u ij for the same pair ij is defined 7 as r ij /(α i α j ) -1 6 . The parameter a for all atoms is set as 0.333 Å-1 , which is the default value of MPID. Other damping functions between multipole moments can also be found in table I of reference.