• IRPJAST

IRPJAST

CHAMP's Jastrow factor computation using the IRPF90 method

Original equation:

$$ \sum_{i=2}^{Ne} \sum_{j=1}^i \sum_{pkl} \sum_a^{Nn} c_{apkl}\, r_{ij}^k\, ( R_{ia}^l + R_{ja}^l) ( R_{ia} R_{ja})^m $$

Expanding, one obtains:

$$ \sum_{i=2}^{Ne} \sum_{j=1}^i \sum_{pkl} \sum_a^{Nn} c_{apkl} R_{ia}^{p-k-l}\, r_{ij}^k\, R_{ja}^{p-k+l} + c_{apkl} R_{ia}^{p-k+l}\, r_{ij}^k\, R_{ja}^{p-k-l} $$

The equation is symmetric if we exchange $i$ and $j$, so we can rewrite

$$ \sum_{i=1}^{Ne} \sum_{j=1}^{Ne} \sum_{pkl} \sum_a^{Nn} c_{apkl} R_{ia}^{p-k-l}\, r_{ij}^k\, R_{ja}^{p-k+l} $$

If we reshape $R_{ja}^p$ as a matrix $R_{j,al}$ of size $N_e \times N_n(N_c+1)$, for every $k$, we can pre-compute the matrix product

$$ C_{i,al}^{k} = \sum_j r_{ij}^k\, R_{i,al} $$ which can be computed efficiently with BLAS. We can express the total Jastrow as:

$$ \sum_{i=1}^{Ne} \sum_{pkl} \sum_a^{Nn} c_{apkl} R_{ia}^{p-k-l}\, C_{i,a(p-k+l)}^k $$