R^2通常的定义是 R^2=1-(SSre/SStot)
    SSre 是 the sum of the squares of the distances of the points from the fit.
    SStot是 the sum of the squares of the distances of the points from a horizontal line through the mean of all Y values.
    据此，可以在R语言中实现的代码如下：

#首先得到非线性拟合的结果
#假设V为实测的因变量值
Reg <- nls(...)

#计算SSre
# 得到fitted value
fit<-fitted(Reg)
SSre<-sum((V-fit)^2)
#或者
SSre<-sum(residuals(Reg)^2)

#计算SStot
SStot<-sum((V-mean(V)）^2)

#计算R^2
R2=1-SSre/SStot