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\begin{eqnarray}
\sf 
\vec{r}_{n+1} & = & \vec{r}_n + \frac{h}{m} \vec{p}_n +
     \frac{h^2}{2m} \vec{F}_n + \frac{h}{3} \vec{r}_n \frac{\dot{V}_n}{V_n} \nonumber\\
V_{n+1} & = & V_n + h\dot{V}_n + \frac{h^2}{2} \ddot{V}_n \\
\vec{p}_{n+1} & = & \vec{p}_n + \frac{h}{2}(\vec{F}_n + \vec{F}_{n+1}) -
     \frac{h}{3} \vec{p}_n \frac{\dot{V}_n}{V_n} \nonumber\\
\dot{V}_{n+1} & = & \dot{V}_n + \frac{h}{2}(\ddot{V}_n + \ddot{V}_{n+1}) \nonumber
\end{eqnarray}
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