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<LI><A ID="tex2html177"
HREF="node13.html#SECTION00046010000000000000">3.6.0.1 Practicalities</A>
<LI><A ID="tex2html178"
HREF="node13.html#SECTION00046020000000000000">3.6.0.2 How to use the i-PI inteface</A>
<LI><A ID="tex2html179"
HREF="node13.html#SECTION00046030000000000000">3.6.0.3 Advanced i-PI usage</A>
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<H2><A ID="SECTION00046000000000000000">
3.6 Socket interface with i-PI</A>
</H2>
<P>
The i-PI universal force engine performs advanced Molecular Dynamics
(MD) (such as Path Integral Molecular Dynamics, Thermodynamic
Integration, Suzuki-Chin path integral, Multiple Time Step molecular
dynamics) and other force related computations (see <kbd><A ID="tex2html1"
HREF="ipi-code.org">ipi-code.org</A></kbd>for more information about i-PI).
<P>
<TT>PWscf</TT> users wishing to learn how to use i-PI should refer to the i-PI website.
<P>
<H4><A ID="SECTION00046010000000000000">
3.6.0.1 Practicalities</A>
</H4>
The communication between <TT>PWscf</TT> and i-PI relies on a socket
interface. This allows running i-PI and <TT>PWscf</TT> on different computers
provided that the two computers have an Internet
connection. Basically, i-PI works as a server waiting for a connection
of a suitable software (for example <TT>PWscf</TT>). When this happens, i-PI
injects atomic positions and cell parameters into the software, that
will return forces and stress tensor to i-PI.
<P>
The file containing the interface is <code>run_driver.f90</code>. The files
<code>socket.c</code> and <code>fsocket.f90</code> provide the necessary
infrastructure to the socket interface.
<P>
<H4><A ID="SECTION00046020000000000000">
3.6.0.2 How to use the i-PI inteface</A>
</H4>
Since the communication goes through the Internet, the
<TT>PWscf</TT> instance needs to know the address of the i-PI server that can
be specified with the command line option <code>--ipi</code> (or
<code>-ipi</code>) followed by the address of the computer running i-PI and
the port number where i-PI is listening, e.g.
<PRE>
pw.x --ipi localhost:3142 -in pw.input > pw.out
</PRE>
If i-PI and <TT>PWscf</TT> are running on the same machine, a UNIX socket is
preferable since allows faster communications, e.g.
<PRE>
pw.x --ipi socketname:UNIX -in pw.input > pw.out
</PRE>
In the last case, <code>UNIX</code> is a keyword that tells to <TT>PWscf</TT> to
look for an UNIX socket connection instead of an INET one. More
extensive examples and tutorials can be found at <kbd><A ID="tex2html2"
HREF="ipi-code.org">ipi-code.org</A></kbd>.
<P>
The <TT>PWscf</TT> input file must contain all the information to
perform a single point calculation (<code>calculation = "scf"</code>) which
are also used to initialize the <TT>PWscf</TT> run. Thus, it is important
that the <TT>PWscf</TT> input contains atomic positions and cell parameters
which are as close as possible to those specified in the i-PI input.
<P>
<H4><A ID="SECTION00046030000000000000">
3.6.0.3 Advanced i-PI usage</A>
</H4>
<P>
For more advanced users, calculation flags can be changed
on-the-fly by parsing a single binary-encoded integer to QE through
the i-PI socket. That gives users the flexibility to define what
properties to be calculated. For example, if only a single SCF cycle
is needed, traditionally <code>run_driver.f90</code> would be set to
calculate not only the potential energy, but also forces, stresses
and initialize g-vectors. With the binary-integer encoded flags, now
one can turn flags on and off as necessary to speed up their code flow.
<P>
The sequence of flags that is currently accepted is: SCF,
forces, stresses, variable-cell and ensembles. The latter is only
available if QE has been compiled against BEEF-vdW XC. For a SCF and
forces-only calculation, that would corresponds to a <code>11000</code>
sequence, which has a <code>24</code> decimal representation. The QE side
of the i-PI socket expects the equivalent-decimal<code>+1</code>; therefore,
for a <code>11000</code> calculation, the integer <code>25</code> would have to be
parsed to the <code>driver_init</code> subroutine in <code>run_driver.f90</code>. If
any number less-than or equal-to <code>1</code> is parsed to QE, it falls back
to its standard i-PI mode.
<P>
Currently, the QE i-PI interface can only reside in
three different states: "NEEDINIT", "READY" or "HAVEDATA". Whenever
the socket sends a "STATUS" message to QE, it responds back with its
current status. A simple calculation sequence of events would be: (1) an
"STATUS" message is received, QE sends back "NEEDINIT", (2) an "INIT"
message is received, QE waits for three data packets, (i) an integer
that identifies the client on the other side of the socket, (ii) the flag-encoded integer
mentioned above, which can be used to change calculation
settings, and (iii) an initialization string. QE then changes its status
to "READY". (3) The server sends a "POSDATA" message and QE then expects
a sequence of variables depending on the calculation settings; the default
being: a 3-by-3 matrix with cell and 3-by-3 marix with its inverse (if
<code>lmovecell</code> is <code>.TRUE.</code>) and a (# of atoms)-by-3 position matrix. QE
proceeds and computes all active properties (e.g. SCF, forces, stresses,
etc.) and change its status to "HAVEDATA" and expects a (4) "GETFORCE"
message from the socket. Once it is received, (5) QE sends back (i) a
float with the potential energy, (ii) an integer with the total number of
atoms, (iii) a (# of atoms)-by-3 matrix with forces (if <code>lforce</code> is
<code>.TRUE.</code>), (iv) a 9-element-virial tensor (if <code>lstres</code> is
<code>.TRUE.</code>). QE goes back to "NEEDINIT" status. The other side of socket
should be able to compute new positions and cell coordinates (if <code>lmovecell</code>
is <code>.TRUE.</code>) and start the cycle again from (1).
<P>
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