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$$
{R_G}^2 = \frac{1}{M}\sum_{i}^{N}{m_{i}\left( x_{i} -
\frac{1}{M}\sum_{j}^{N}{m_{j}x_{j}} \right)^{2}}
$$

$$
 E = K\left( R_G - R_{G0} \right)^{2}
$$

$$
F_{i} = 2K\frac{m_{i}}{M}\left( 1-\frac{R_{G0}}{R_G}
\right)\left( x_{i} - \frac{1}{M}\sum_{j}^{N}{m_{j}x_{j}} \right)
$$

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