Add more fHS-WCA tests

[kjs73]

This adds some more cpp tests for the fHS-WCA potential. The implementation wrapped is compared to an independent basic cpp implementation. A function to plot the fHS-WCA pair-potential is added to the interaction struct. Also the energy is made strictly non-negative to avoid numerical problems.

[js850]

what is fHS-WCA?

[smcantab]

fast HS-WCA, it's not really a HS + WCA. We don't take the sqrt in the denominator.

On Mon, Dec 1, 2014 at 5:35 PM, Jacob Stevenson notifications@github.com wrote:

what is fHS-WCA?

— Reply to this email directly or view it on GitHub https://github.com/pele-python/pele/pull/104#issuecomment-65102387.

[coveralls]

Coverage remained the same when pulling e94707b9b3b2a6b492a777df1f80f5d35a0fd4e4 on kjs73:hs_wca_again into afe6526fb5db9bc2e23bfc9dadf19d9f4da04813 on pele-python:master.

[kjs73]

The potential now looks like this: V_\text{sfHS-WCA}(r^2) = 0 \text{ if } r \geq rS V\text{sfHS-WCA}(r^2) = V\text{fHS-WCA}(r^2) \text{ if } r\times < r < rS V\text{sfHS-WCA}(r^2) = E\times - (\sqrt{r^2} - r\times)G\times \text{ if } r \leq r\times Here: E\times = V\text{fHS-WCA}(r\times) G\times = \text{grad}V_\text{fHS-WCA} And: r_H : sum of hard radii r_S = (1 + \alpha) * rH r\times = r_H + \delta An example plot comes below.

[kjs73]

[js850]

It looks OK to me.

[coveralls]

Coverage remained the same when pulling 918671696e90ea7f4ae19887ac3a002ca781e06f on kjs73:hs_wca_again into b0e1a8a290a140272c5f3096f989054ffe27f95b on pele-python:master.

[coveralls]

Coverage increased (+0.04%) when pulling 918671696e90ea7f4ae19887ac3a002ca781e06f on kjs73:hs_wca_again into b0e1a8a290a140272c5f3096f989054ffe27f95b on pele-python:master.

[kjs73]

Thank you! @smcantab are you happy with this version?