denormalized_density_summation Subroutine

    subroutine denormalized_density_summation(particle, pairs, n)
        type(particle_type), intent(inout) :: particle      !! 区域
        type(nnps_pairs_type), intent(in) :: pairs          !! 粒子对
        integer, intent(in) :: n                            !! 实弹性粒子数 \( N_{real} \)
        integer :: i

        particle%rho(:n) = skf_obj%selfden*particle%mass(:n)              ! 计算自身对密度的影响，并初始化当前步长密度值

        do i = 1, pairs%len                        ! 计算密度求和
            associate (ip => pairs%items(2*i - 1), jp => pairs%items(2*i))

                if (ip <= n .and. jp <= n) then
                    if (particle%itype(ip) /= particle%itype(jp)) cycle
                    particle%rho(ip) = particle%rho(ip) + particle%mass(jp)*pairs%wdwdx(1, i)
                    particle%rho(jp) = particle%rho(jp) + particle%mass(ip)*pairs%wdwdx(1, i)
                end if

            end associate
        end do

    end subroutine denormalized_density_summation