diff --git a/build_things.sh b/build_things.sh
index 83800fa9..71314116 100755
--- a/build_things.sh
+++ b/build_things.sh
@@ -172,6 +172,7 @@ refine_structure
 thermal_conductivity
 thermal_conductivity_2023
 anharmonic_free_energy
+remap_forceconstants
 "
 
 # only when we have cgal
diff --git a/src/remap_forceconstant/Makefile b/src/remap_forceconstant/Makefile
new file mode 100644
index 00000000..09e6bbf3
--- /dev/null
+++ b/src/remap_forceconstant/Makefile
@@ -0,0 +1,31 @@
+include Makefile.inc
+CODE = remap_forceconstant
+PROG = ../../build/$(CODE)/$(CODE)
+OBJECT_PATH=../../build/$(CODE)/
+
+OBJS = $(OBJECT_PATH)main.o $(OBJECT_PATH)options.o 
+
+LPATH = -L../../lib $(blaslapackLPATH) $(incLPATHhdf) $(incLPATHmpi)
+IPATH = -I../../inc/libolle -I../../inc/libflap $(blaslapackIPATH) $(incIPATHhdf) $(incIPATHmpi)
+LIBS = ../../lib/libolle.a -lflap $(blaslapackLIBS) $(incLIBShdf) $(incLIBSmpi)
+
+#OPT = -O0 -fbacktrace -fcheck=all
+F90 = $(FC) $(LPATH) $(IPATH) $(MODULE_FLAG) $(OBJECT_PATH) 
+F90FLAGS = $(OPT) $(MODS) $(LIBS)
+
+all: $(PROG)
+
+$(PROG): $(OBJS)
+	$(F90) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
+
+clean:
+	rm -f $(PROG) $(OBJS) *.mod
+
+.f90.o:
+	$(F90) $(F90FLAGS) -c $< $(LIBS)
+
+$(OBJECT_PATH)main.o: $(OBJECT_PATH)options.o 
+	$(F90) $(F90FLAGS) -c main.f90 -o $@
+$(OBJECT_PATH)options.o:
+	$(F90) $(F90FLAGS) -c options.f90 -o $@
+
diff --git a/src/remap_forceconstant/main.f90 b/src/remap_forceconstant/main.f90
new file mode 100644
index 00000000..968b4de5
--- /dev/null
+++ b/src/remap_forceconstant/main.f90
@@ -0,0 +1,294 @@
+#include "precompilerdefinitions"
+program remap_forceconstant
+!!{!src/remap_forceconstant/manual.md!}
+use konstanter, only: flyt, lo_sqtol, lo_freqtol
+use gottochblandat, only: lo_invert3x3matrix, open_file, lo_does_file_exist, lo_frobnorm
+use mpi_wrappers, only: lo_mpi_helper
+use lo_memtracker, only: lo_mem_helper
+use type_crystalstructure, only: lo_crystalstructure
+use type_forceconstant_secondorder, only: lo_forceconstant_secondorder
+use type_forceconstant_thirdorder, only: lo_forceconstant_thirdorder
+use type_forceconstant_fourthorder, only: lo_forceconstant_fourthorder
+use options, only: lo_opts
+use lo_memtracker, only: lo_mem_helper
+
+
+implicit none
+
+type(lo_opts) :: opts
+type(lo_crystalstructure) :: uc, nc
+type(lo_forceconstant_secondorder) :: fc, fcn
+type(lo_forceconstant_thirdorder) :: fct, fctn
+type(lo_forceconstant_fourthorder) :: fcf !,fcfn
+type(lo_mpi_helper) :: mw
+type(lo_mem_helper) :: mem
+integer :: i,j,k,l,u,ii
+real(flyt), dimension(3) :: v0
+real(flyt), dimension(3, 3) :: tm
+real(flyt), dimension(:, :, :), allocatable :: dfc1, dfc2
+!
+call opts%parse()
+call mem%init()
+call mw%init()
+! Start memory tracker
+call mem%init()
+
+! read files
+call uc%readfromfile('infile.ucposcar')
+call nc%readfromfile('infile.newposcar')
+write (*, *) '... read structures'
+
+if (lo_does_file_exist('infile.forceconstant')) then
+    call fc%readfromfile(uc, 'infile.forceconstant', mem, 1)
+    write (*, *) '... read secondorder forceconstant'
+end if
+!
+if (lo_does_file_exist('infile.forceconstant_thirdorder')) then
+    call fct%readfromfile(uc, 'infile.forceconstant_thirdorder')
+    write (*, *) '... read thirdorder forceconstant'
+end if
+!
+if (lo_does_file_exist('infile.forceconstant_fourthorder')) then
+    call fcf%readfromfile(uc, 'infile.forceconstant_fourthorder')
+    write (*, *) '... read fourthorder forceconstant'
+end if
+
+! check for a transformation matrix
+if (lo_does_file_exist('infile.transformation')) then
+    write (*, *) 'Trouble: I should fix this'
+    stop
+    u = open_file('in', 'infile.transformation')
+    do i = 1, 3
+        read (u, *) tm(i, :)
+    end do
+    write (*, *) '... read a transformation matrix:'
+    do i = 1, 3
+        write (*, "(3(2X,F11.8))") tm(i, :)
+    end do
+else
+    ! use the identity transformation
+    tm(1, :) = [1.0_flyt, 0.0_flyt, 0.0_flyt]
+    tm(2, :) = [0.0_flyt, 1.0_flyt, 0.0_flyt]
+    tm(3, :) = [0.0_flyt, 0.0_flyt, 1.0_flyt]
+end if
+
+! Transform the original cell
+tm = transpose(tm)
+uc%latticevectors = matmul(uc%latticevectors, tm)
+uc%inv_latticevectors = lo_invert3x3matrix(uc%latticevectors)
+! need to transpose here, otherwise it does not work
+tm = transpose(tm)
+
+! Transform and remap the forceconstants
+if (lo_does_file_exist('infile.forceconstant')) then
+    ! transform
+    do i = 1, fc%na
+        do j = 1, fc%atom(i)%n
+            fc%atom(i)%pair(j)%lv1 = matmul(tm, fc%atom(i)%pair(j)%lv1)
+            fc%atom(i)%pair(j)%lv2 = matmul(tm, fc%atom(i)%pair(j)%lv2)
+            fc%atom(i)%pair(j)%r = matmul(tm, fc%atom(i)%pair(j)%r)
+            fc%atom(i)%pair(j)%m = rotate(tm, fc%atom(i)%pair(j)%m)
+        end do
+    end do
+    ! remap
+    write (*, *) 'Remapping, cutoff:', fc%cutoff
+    call nc%classify('supercell', uc)
+    call fc%remap(uc, nc, fcn)
+    ! print forceconstant file
+    call fcn%writetofile(nc, 'outfile.forceconstant_remapped')
+    write (*, *) 'remapped the second order force constants'
+    ! print this in LAMMPS-compliant format.
+    if (opts%lammps) then
+        ! print the output in a lammps-friendly way,
+        ! first we write the equilibrium positions:
+        u = open_file('out', 'outfile.lammps_eqpos')
+        do i = 1, nc%na
+            !v0=nc%r(:,i)
+            !do j=1,3
+            !    if ( abs(v0(j)) .lt. lo_sqtol ) v0(j)=1.0_flyt
+            !enddo
+            !write(u,*) matmul(nc%latticevectors,v0)
+            write (u, *) nc%rcart(:, i) !-matmul(nc%latticevectors,[1_flyt,1_flyt,1_flyt])
+        end do
+        close (u)
+        ! second is a list of the forceconstants, only the small cell ones
+        l = 0
+        do i = 1, fc%na
+            l = l + fc%atom(i)%n
+        end do
+        lo_allocate(dfc1(3, 3, l))
+
+        l = 0
+        do i = 1, fc%na
+        do j = 1, fc%atom(i)%n
+            l = l + 1
+            dfc1(:, :, l) = fc%atom(i)%pair(j)%m
+        end do
+        end do
+
+        u = open_file('out', 'outfile.lammps_pairfc')
+        ! number of distinct forceconstants
+        write (u, *) l
+        ! all the forceconstants
+        do i = 1, l
+        do j = 1, 3
+            write (u, *) dfc1(j, :, i)
+        end do
+        end do
+        close (u)
+
+        ! next print a map of all pairs.
+        u = open_file('out', 'outfile.lammps_pairmap')
+        ! how many atoms, cutoff
+        write (u, *) fcn%na, fcn%cutoff
+        ! the number of neighbours for each atom
+        do i = 1, fcn%na
+            write (u, *) i, fcn%atom(i)%n
+        end do
+        ! now massive loop over all pairs
+        do i = 1, fcn%na
+        do j = 1, fcn%atom(i)%n
+            ! index to atom 2
+            k = fcn%atom(i)%pair(j)%i2
+            ! vector to atom 2
+            v0 = fcn%atom(i)%pair(j)%r
+            ! which forceconstant?
+            ii = -1
+            do l = 1, size(dfc1, 3)
+                if (lo_frobnorm(dfc1(:, :, l) - fcn%atom(i)%pair(j)%m) .lt. lo_sqtol) then
+                    ii = l
+                    exit
+                end if
+            end do
+            write (u, "(4(1X,I5),3(1X,F18.10))") i, j, k, ii, v0
+        end do
+        end do
+        close (u)
+
+        ! get the harmonic stuff
+        call fcn%initialize_cell(nc, uc, fc, 0.0_flyt, .true., .false., lo_freqtol, mw=mw)
+        u = open_file('out', 'outfile.commensurate_mode_frequencies')
+        do i = 1, fcn%na*3
+            write (u, *) fcn%omega(i)
+        end do
+        close (u)
+    end if
+end if
+
+if (lo_does_file_exist('infile.forceconstant_thirdorder')) then
+    ! transform
+    do i = 1, fct%na
+        do j = 1, fct%atom(i)%n
+            fct%atom(i)%triplet(j)%lv1 = matmul(tm, fct%atom(i)%triplet(j)%lv1)
+            fct%atom(i)%triplet(j)%lv2 = matmul(tm, fct%atom(i)%triplet(j)%lv2)
+            fct%atom(i)%triplet(j)%lv3 = matmul(tm, fct%atom(i)%triplet(j)%lv3)
+            fct%atom(i)%triplet(j)%rv1 = matmul(tm, fct%atom(i)%triplet(j)%rv1)
+            fct%atom(i)%triplet(j)%rv2 = matmul(tm, fct%atom(i)%triplet(j)%rv2)
+            fct%atom(i)%triplet(j)%rv3 = matmul(tm, fct%atom(i)%triplet(j)%rv3)
+            !
+            fct%atom(i)%triplet(j)%m = rotate3(tm, fct%atom(i)%triplet(j)%m)
+        end do
+    end do
+    ! remap
+    call fct%remap(uc, nc, fctn)
+    call fctn%writetofile(nc, 'outfile.forceconstant_thirdorder_remapped')
+    write (*, *) 'remapped the third order force constants'
+end if
+
+!if ( lo_does_file_exist('infile.forceconstant_fourthorder') ) then
+!    ! transform
+!    do i=1,fcf%na
+!        do j=1,fcf%atom(i)%n
+!            fcf%atom(i)%quartet(j)%lv1=matmul(tm,fcf%atom(i)%quartet(j)%lv1)
+!            fcf%atom(i)%quartet(j)%lv2=matmul(tm,fcf%atom(i)%quartet(j)%lv2)
+!            fcf%atom(i)%quartet(j)%lv3=matmul(tm,fcf%atom(i)%quartet(j)%lv3)
+!            fcf%atom(i)%quartet(j)%lv4=matmul(tm,fcf%atom(i)%quartet(j)%lv4)
+!            fcf%atom(i)%quartet(j)%rv1=matmul(tm,fcf%atom(i)%quartet(j)%rv1)
+!            fcf%atom(i)%quartet(j)%rv2=matmul(tm,fcf%atom(i)%quartet(j)%rv2)
+!            fcf%atom(i)%quartet(j)%rv3=matmul(tm,fcf%atom(i)%quartet(j)%rv3)
+!            fcf%atom(i)%quartet(j)%rv4=matmul(tm,fcf%atom(i)%quartet(j)%rv4)
+!            !
+!            fcf%atom(i)%quartet(j)%m=rotate4(tm,fcf%atom(i)%quartet(j)%m)
+!            !
+!        enddo
+!    enddo
+!    ! remap
+!    call fcf%remap(uc,nc,fcfn)
+!    call fcfn%writetofile(nc,'outfile.forceconstant_fourthorder_remapped')
+!    write(*,*) 'remapped the fourth order force constants'
+!endif
+
+contains
+!
+function rotate(op, m) result(n)
+    real(flyt), dimension(3, 3), intent(in) :: op, m
+    real(flyt), dimension(3, 3) :: n
+    !
+    integer :: i, j, ii, jj
+    !
+    n = 0.0_flyt
+    do j = 1, 3
+    do i = 1, 3
+        do jj = 1, 3
+        do ii = 1, 3
+            n(i, j) = n(i, j) + m(ii, jj)*op(i, ii)*op(j, jj)
+        end do
+        end do
+    end do
+    end do
+    !
+end function
+!
+function rotate3(op, m) result(n)
+    real(flyt), dimension(3, 3, 3), intent(in) :: m
+    real(flyt), dimension(3, 3), intent(in) :: op
+    real(flyt), dimension(3, 3, 3) :: n
+    !
+    integer :: i, j, k, ii, jj, kk
+    !
+    n = 0.0_flyt
+    do i = 1, 3
+    do j = 1, 3
+    do k = 1, 3
+        do ii = 1, 3
+        do jj = 1, 3
+        do kk = 1, 3
+            n(i, j, k) = n(i, j, k) + m(ii, jj, kk)*op(i, ii)*op(j, jj)*op(k, kk)
+        end do
+        end do
+        end do
+    end do
+    end do
+    end do
+    !
+end function
+!
+function rotate4(op, m) result(n)
+    real(flyt), dimension(3, 3, 3, 3), intent(in) :: m
+    real(flyt), dimension(3, 3), intent(in) :: op
+    real(flyt), dimension(3, 3, 3, 3) :: n
+    !
+    integer :: i, j, k, l, ii, jj, kk, ll
+    !
+    n = 0.0_flyt
+    do i = 1, 3
+    do j = 1, 3
+    do k = 1, 3
+    do l = 1, 3
+        do ii = 1, 3
+        do jj = 1, 3
+        do kk = 1, 3
+        do ll = 1, 3
+            n(i, j, k, l) = n(i, j, k, l) + m(ii, jj, kk, ll)*op(i, ii)*op(j, jj)*op(k, kk)*op(l, ll)
+        end do
+        end do
+        end do
+        end do
+    end do
+    end do
+    end do
+    end do
+    !
+end function
+!
+end program
diff --git a/src/remap_forceconstant/manual.md b/src/remap_forceconstant/manual.md
new file mode 100644
index 00000000..15a3c645
--- /dev/null
+++ b/src/remap_forceconstant/manual.md
@@ -0,0 +1,18 @@
+author: Olle Hellman
+display: none
+graph: none
+propname: remap forceconstant
+propnamelink: <a href="../program/remap_forceconstant.html">remap forceconstant</a>
+{!man/remap_forceconstant.md!}
+
+### Longer summary
+
+@todo Actually write this
+
+### What should be here
+
+* explain how my force constant structure works, and how it can be remapped.
+* emphasize that there is no particular cell attached to a forceconstant.
+* one simple example, fcc primitive to fcc conventional.
+* one more annoying, take the one with fcc as a hexagonal cell.
+
diff --git a/src/remap_forceconstant/options.f90 b/src/remap_forceconstant/options.f90
new file mode 100644
index 00000000..0957f904
--- /dev/null
+++ b/src/remap_forceconstant/options.f90
@@ -0,0 +1,58 @@
+#include "precompilerdefinitions"
+module options
+use konstanter, only: lo_status, lo_author, lo_version, lo_licence
+use flap, only: command_line_interface
+implicit none
+private
+public :: lo_opts
+
+type lo_opts
+    logical :: lammps
+contains
+    procedure :: parse
+end type
+
+contains
+
+!> parse the command line arguments and set defaults
+subroutine parse(opts)
+    !> the options
+    class(lo_opts), intent(out) :: opts
+    !> the helper parser
+    type(command_line_interface) :: cli
+
+    logical :: dumlog
+
+    call cli%init(progname='remap_forceconstant', &
+                  authors=lo_author, &
+                  version=lo_version, &
+                  license=lo_licence, &
+                  help='Usage: ', &
+                  description='Takes a set of forceconstants and a unit cell and outputs them for a different unit cell. The lattices must be equivalent, and if the new lattice is rotated with respect to old the rotation matrix has to be specified.', &
+                  examples=["remap_forceconstant"], &
+                  epilog=new_line('a')//"...")
+
+    call cli%add(switch='--lammps', hidden=.true., &
+                 help='Generate output compatible with my LAMMPS thing.', &
+                 required=.false., act='store_true', def='.false.', error=lo_status)
+    if (lo_status .ne. 0) stop
+
+    cli_manpage
+    ! actually parse it
+    call cli%parse(error=lo_status)
+    if (lo_status .ne. 0) stop
+
+    dumlog = .false.
+    call cli%get(switch='--manpage', val=dumlog)
+    if (dumlog) then
+        call cli%save_man_page(trim(cli%progname)//'.1')
+        call cli%save_usage_to_markdown(trim(cli%progname)//'.md')
+        write (*, *) 'Wrote manpage for "'//trim(cli%progname)//'"'
+        stop
+    end if
+
+    call cli%get(switch='--lammps', val=opts%lammps)
+
+end subroutine
+
+end module