Adding kspace_modify diff option to select the PPPM differentiation method.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@8040 f3b2605a-c512-4ea7-a41b-209d697bcdaa

doc/kspace_modify.html

@@ -17,7 +17,7 @@
 </PRE>
 <UL><LI>one or more keyword/value pairs may be listed 
 
-<LI>keyword = <I>mesh</I> or <I>order</I> or <I>gewald</I> or <I>slab</I> 
+<LI>keyword = <I>mesh</I> or <I>order</I> or <I>gewald</I> or <I>slab</I> or <I>compute</I> or <I>diff</I> 
 
 <PRE>  <I>mesh</I> value = x y z
     x,y,z = PPPM FFT grid size in each dimension
@@ -30,6 +30,7 @@
     volfactor = ratio of the total extended volume used in the
       2d approximation compared with the volume of the simulation domain
   <I>compute</I> value = <I>yes</I> or <I>no</I> 
+  <I>diff</I> value = <I>ik</I> or <I>ad</I> 
 </PRE>
 
 </UL>
@@ -111,6 +112,14 @@ defining a <A HREF = "kspace_style.html">kspace_style</A>, but a Kspace-compatib
 <A HREF = "pair_style.html">pair_style</A> requires a kspace_style to be defined.
 This keyword gives you that option.
 </P>
+<P>The <I>diff</I> keyword specifies the differentiation scheme used by the
+PPPM method to compute forces on particles given electrostatic 
+potentials on the PPPM mesh. The ik approach is the default that 
+performs differentiation in kspace, but requires 3 FFTs to transfer
+the computed fields back to real space. The analytic differentiation,
+or ad approach requires only 1 FFT, but requires a somewhat bigger
+PPPM mesh to achieve the same accuracy as the ik approach.
+</P>
 <P><B>Restrictions:</B> none
 </P>
 <P><B>Related commands:</B>
@@ -120,7 +129,7 @@ This keyword gives you that option.
 <P><B>Default:</B>
 </P>
 <P>The option defaults are mesh = 0 0 0, order = 5, force = -1.0, gewald
-= 0.0, slab = 1.0, and compute = yes.
+= 0.0, slab = 1.0, compute = yes, and diff = ik.
 </P>
 <HR>
 

doc/kspace_modify.txt

@@ -13,7 +13,7 @@ kspace_modify command :h3
 kspace_modify keyword value ... :pre
 
 one or more keyword/value pairs may be listed :ulb,l
-keyword = {mesh} or {order} or {gewald} or {slab} :l
+keyword = {mesh} or {order} or {gewald} or {slab} or {compute} or {diff} :l
   {mesh} value = x y z
     x,y,z = PPPM FFT grid size in each dimension
   {order} value = N
@@ -24,7 +24,8 @@ keyword = {mesh} or {order} or {gewald} or {slab} :l
   {slab} value = volfactor
     volfactor = ratio of the total extended volume used in the
       2d approximation compared with the volume of the simulation domain
-  {compute} value = {yes} or {no} :pre
+  {compute} value = {yes} or {no} 
+  {diff} value = {ik} or {ad} :pre
 :ule
 
 [Examples:]
@@ -105,6 +106,14 @@ defining a "kspace_style"_kspace_style.html, but a Kspace-compatible
 "pair_style"_pair_style.html requires a kspace_style to be defined.
 This keyword gives you that option.
 
+The {diff} keyword specifies the differentiation scheme used by the
+PPPM method to compute forces on particles given electrostatic 
+potentials on the PPPM mesh. The ik approach is the default that 
+performs differentiation in kspace, but requires 3 FFTs to transfer
+the computed fields back to real space. The analytic differentiation,
+or ad approach requires only 1 FFT, but requires a somewhat bigger
+PPPM mesh to achieve the same accuracy as the ik approach.
+
 [Restrictions:] none
 
 [Related commands:]
@@ -114,7 +123,7 @@ This keyword gives you that option.
 [Default:]
 
 The option defaults are mesh = 0 0 0, order = 5, force = -1.0, gewald
-= 0.0, slab = 1.0, and compute = yes.
+= 0.0, slab = 1.0, compute = yes, and diff = ik.
 
 :line