Index: trunk/LMDZ.MARS/libf/aeronomars/calchim_mod.F90
===================================================================
--- trunk/LMDZ.MARS/libf/aeronomars/calchim_mod.F90	(revision 2162)
+++ trunk/LMDZ.MARS/libf/aeronomars/calchim_mod.F90	(revision 2164)
@@ -3,4 +3,5 @@
   IMPLICIT NONE
 
+  INTEGER, SAVE :: ichemistry ! compute chemistry every ichemistry physics step
   REAL,SAVE,ALLOCATABLE :: zdqchim(:,:,:) ! Tendancy on pq due to photochemistry
   REAL,SAVE,ALLOCATABLE :: zdqschim(:,:) ! Tendancy on qsurf due to photochemistry
Index: trunk/LMDZ.MARS/libf/phymars/conf_phys.F
===================================================================
--- trunk/LMDZ.MARS/libf/phymars/conf_phys.F	(revision 2162)
+++ trunk/LMDZ.MARS/libf/phymars/conf_phys.F	(revision 2164)
@@ -44,4 +44,5 @@
      &                      ini_scatterers,tauvis
       use datafile_mod, only: datadir
+      use calchim_mod, only: ichemistry
 
       IMPLICIT NONE
@@ -688,9 +689,15 @@
          write(*,*) "tituscap",tituscap
                      
-
+! Chemistry:
          write(*,*) "photochemistry: include chemical species"
          photochem=.false. ! default value
          call getin("photochem",photochem)
          write(*,*) " photochem = ",photochem
+         
+         write(*,*) "Compute chemistry (if photochem is .true.)",
+     &   "every ichemistry physics step (default: ichemistry=1)"
+         ichemistry=1
+         call getin("ichemistry",ichemistry)
+         write(*,*) " ichemistry = ",ichemistry
 
 
Index: trunk/LMDZ.MARS/libf/phymars/physiq_mod.F
===================================================================
--- trunk/LMDZ.MARS/libf/phymars/physiq_mod.F	(revision 2162)
+++ trunk/LMDZ.MARS/libf/phymars/physiq_mod.F	(revision 2164)
@@ -15,5 +15,5 @@
 
       use watercloud_mod, only: watercloud, zdqcloud, zdqscloud
-      use calchim_mod, only: calchim, zdqchim, zdqschim
+      use calchim_mod, only: calchim, ichemistry, zdqchim, zdqschim
       use watersat_mod, only: watersat
       use co2condens_mod, only: co2condens
@@ -1615,4 +1615,7 @@
          IF (photochem) then
 
+           if (modulo(icount-1,ichemistry).eq.0) then
+           ! compute chemistry every ichemistry physics step
+
 !           dust and ice surface area
             call surfacearea(ngrid, nlayer, naerkind,
@@ -1627,4 +1630,6 @@
      $                   zdqcloud,zdqscloud,tau(:,1),co2ice,
      $                   pu,pdu,pv,pdv,surfdust,surfice)
+
+            endif ! of if (modulo(icount-1,ichemistry).eq.0)
 
            ! increment values of tracers: