second derivatives from SPLIE2
natural spline assumed
    0.000000    0.101434    0.074264    0.081509    0.079698
    0.000000    0.405735    0.297057    0.326037    0.318791
    0.000000    0.912905    0.668378    0.733586    0.717278
    0.000000    1.622942    1.188229    1.304150    1.275165
    0.000000    2.535847    1.856607    2.037733    1.992452

actual second derivatives
    0.080000    0.080000    0.080000    0.080000    0.080000
    0.320000    0.320000    0.320000    0.320000    0.320000
    0.720000    0.720000    0.720000    0.720000    0.720000
    1.280000    1.280000    1.280000    1.280000    1.280000
    2.000000    2.000000    2.000000    2.000000    2.000000