{\delta }_{p}=\sqrt{{x}_{i}{x}_{j}{V}_{ij}}=\sqrt{\sum_{i=1}^{N}{x}_{i}^{2}{\delta }_{i}^{2}+2\sum_{i=1}^{N-1}\sum_{j=i+1}^{N}{x}_{i}{x}_{j}{r}_{ij}{\delta }_{j}{\delta }_{i}}