Hello Dr. Gale,

I am trying to fit a LJ potential with coulomb charges for Li-Al-O system. While doing so in GULP, I am facing the following problems,

- I assume the default setting of Lennard 12 6 is “atomab” type. When I use the atomab type setting for Li-Al-O system after converting epsilon and sigma to A and B by using 4
*epsilon*sigma^12 and 4*epsilon*sigma^6 respectively, the system stabilizes but drastically changes values of A and B. Whereas, when I use the “epsilon” setting and put in the corresponding values of epsilon and sigma directly , the system shows that the cell parameter has fallen below allowable limit. I checked my conversions of A,B and epsilon, sigma and they look correct. So my question is why am I getting different results by using A, B or epsilon and sigma. I have to use epsilon and sigma setting because, if I use A and B then it is not possible to obtain the epsilon and sigma from A_ij and B_ij when i is not equal to J. There are 2 equations (A and B) but 4 variables (sigma_i sigma_j, epsilon_i and epsilon_j). Also after one of two iterations of optimization with epsilon setting, some epsilon values become negative. Is this possible realistically?

2.I have taken the LJ parameters for Li-Li, Al-Al and O-O from 3 different papers and I have used the averaging rules given in GULP manual to obtain the Li-AL, Al-O and Li-O interactions. Now, while optimizing would you recommend me to vary only Li-Al, Li-O and Al-O interactions? or should I vary all the 12 parameters (2(epsilon and sigma) X 6 interactions)? Surprisingly when I vary all 12 parameters, the Al-Al and O-O interaction is being changed drastically. Since the like pair interactions (Li-Li, AL-Al, O-O) are well established in literature I was wondering if I can keep them fixed. I am using the data from the stiffness matrix of LiAlO2 for the fitting. I currently have 12 observables and 12 variables. But is it better to have lesser number of variables and more observables? I understand this may not be a direct question from GULP but I just wanted to take your advice from your vast experience for doing this the right way.