SPC/Fw model of water

The SPC/Fw is a flexible variant of the rigid SPC model for water . This model has also been re-parametrised for quantum simulations, adopting the name q-SPC/Fw . The model is given by the intra-molecular component (Eq. 2 of ):

$V^{\mathrm {intra} }={\frac {k_{b}}{2}}\left[\left(r_{\mathrm {OH} _{1}}-r_{\mathrm {OH} }^{\mathrm {eq} }\right)^{2}+\left(r_{\mathrm {OH} _{2}}-r_{\mathrm {OH} }^{\mathrm {eq} }\right)^{2}\right]+{\frac {k_{a}}{2}}\left(\vartheta _{\angle \mathrm {HOH} }-\vartheta _{\angle \mathrm {HOH} }^{\mathrm {eq} }\right)^{2}$

and the inter-molecular component (Eq. 3 of ):

$V^{\mathrm {inter} }=\sum _{ij}^{\mathrm {all~pairs} }\left\{4\epsilon _{ij}\left[\left({\frac {\sigma _{ij}}{R_{ij}}}\right)^{12}-\left({\frac {\sigma _{ij}}{R_{ij}}}\right)^{6}\right]+{\frac {q_{i}q_{j}}{R_{ij}}}\right\}$

The parameters for both of these models are given in the following table (Table I of ):

Model	$k_{b}$	$r_{\mathrm {OH} }^{\mathrm {eq} }$ (Å)	$k_{a}$	$\vartheta _{\angle \mathrm {HOH} }^{\mathrm {eq} }$ (deg)	$\sigma _{\mathrm {OO} }$ (Å)	$\epsilon _{\mathrm {OO} }$ (kcal mol-1)	q(O) (e)	q(H) (e)
SPC/Fw	1059.162	1.012	75.90	113.24	3.165492	0.1554253	-0.82	0.41
q-SPC/Fw	1059.162	1.000	75.90	112.0	3.165492	0.1554252	-0.84	0.42

where the units of $k_{b}$ and $k_{a}$ are kcal.mol^-1Å^-2.