code: public static double volatilityConversionLognormalATMtoNormalATM(double forward, double displacement, double maturity, double lognormalVolatiltiy) {
  		double x = lognormalVolatiltiy * Math.sqrt(maturity / 8);
  		double y = org.apache.commons.math3.special.Erf.erf(x);
  		double normalVol = Math.sqrt(2*Math.PI / maturity) * (forward+displacement) * y;
  
  		return normalVol;
  	}
docs: Exact conversion of displaced lognormal ATM volatiltiy to normal ATM volatility.
  
  @param forward The forward
  @param displacement The displacement (considering a displaced lognormal model, otherwise 0.
  @param maturity The maturity
  @param lognormalVolatiltiy The (implied) lognormal volatility.
  @return The (implied) normal volatility.
  @see <a href="http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2687742">Dimitroff, Fries, Lichtner and Rodi: Lognormal vs Normal Volatilities and Sensitivities in Practice</a>