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    1.622941    1.188230    1.304150    1.275165
    0.000000    2.535847    1.856607    2.037733    1.992453

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