1. The problem statement, all variables and given/known data Hi all! I'm having trouble understanding the implementation of some derivatives in the expression (1) of this article: https://www.ncbi.nlm.nih.gov/pubmed/26248210 How do I implement ∑(ij) ∂ijw ? Thank you all in advance. 2. Relevant equations w is a square matrix(120x120) with periodic boundary conditions. 3. The attempt at a solution I understood it as the second derivative of the matrix w, but it doesn't seem to be correct.