NPOI Version

master

The code is as follows:

private double CalculateRate(double nper, double pmt, double pv, double fv, double type, double guess)
{...
f = Math.Exp(nper * Math.Log(1 + rate));
...}

An early version of POI calculates in the same way:

private double calculateRate(double nper, double pmt, double pv, double fv, doub
...
f = Math.exp(nper * Math.log(1 + rate));
..}

However, the above code is revised:

private double calculateRate(double nper, double pmt, double pv, double fv, doub
...
f = Math.pow(1 + rate, nper);
..}

The patch is as follows:
apache/poi@728c2d3