  The nuclear forces are determined by combining the HAL QCD method and a new
type of source smearing technique. The new smearing is a projection to a space
spanned by the lowest-lying eigenvectors of the free Laplacian operator on a
lattice, which enables efficient calculation of hadron correlators at an
affordable cost by utilizing the hadron-level momentum conservations. We find
that this new approach reduces the statistical and systematic errors in the
resultant nuclear forces.
