# LAMMPS Q&A

Posted by XiLock on September 16, 2020

### 问题

###### Out of range atoms - cannot compute PPPM

Out of range atoms 的根本问题在于分子出现了重叠，所以要检查盒子尺寸是否合理，步长是否合理，是不是预平衡没跑好；或者，是不是没加neigh_modify delay 0 every 1 check yes来及时check。

###### triclinic box skew too large

Fix npt with triclinic boxes does not flip when the tilt factor exceeds +/- 0.5, like fix deform does. Rather it allows an extra delta in the tilt to avoid the box sitting right at 0.5 and flipping back and forth. So it lets the tilt be as big as 0.6 before flipping to -0.4 and vice versa. If you saved a restart when the tilt exceeded 0.5, you can use the “box tilt large” command to allow it to be read back in. Assuming you are not going to deform it more dramatically in the subsequent run, that should be fine.