def get_beads_2_M(sigma, SI=False):
    """
    Computes conversion from beads/sigma^3 to mol/L.

    If SI True, sigma is expected in meters. Otherwise, in Angstroms.
    """
    # Avogadro's number [molecules/mol]
    NA = 6.022E23
    # conversion of meters^3 to L
    m3_2_L = 1E3
    # conversion of meters to Angstroms [A]
    A_2_m = 1E-10
    if not SI:
        sigma *= A_2_m # converts from A to m

    # conversion from beads/sigma^3 to mol/L (M)
    beads_2_M = (NA * sigma**3 * m3_2_L)**(-1)

    return beads_2_M