[cctbxbb] Patterson map data structure

Ten Eyck, Lynn teneyckl at sdsc.edu
Tue Oct 16 10:52:18 PDT 2012

Any Fourier map without an F000 term will analytically have a mean value of 0.


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-----Original message-----
From: Nathaniel Echols <nechols at lbl.gov>
To: cctbx mailing list <cctbxbb at phenix-online.org>
Sent: Tue, Oct 16, 2012 17:22:17 GMT+00:00
Subject: Re: [cctbxbb] Patterson map data structure

On Tue, Oct 16, 2012 at 6:47 AM, Dmytro Guzenko
<Dmytro.Guzenko at pharm.kuleuven.be> wrote:
> rmap = pmap.real_map_unpadded()
> n_real = pmap.n_real()
> unit_cell = pmap.unit_cell()
> grid2cart = maptbx.grid2cart(n_real,unit_cell.orthogonalization_matrix())
> n_points_total = n_real[0]*n_real[1]*n_real[2]
> cart_coord = flex.double(flex.grid(3,n_points_total))
> p_weight = flex.double(n_points_total)
> for plain_ind,grid_point in enumerate(flex.nested_loop(n_real)):
>     cart_coord[0,plain_ind],cart_coord[1,plain_ind],cart_coord[2,plain_ind] = grid2cart(grid_point)
>     p_weight[plain_ind] = rmap[grid_point]

There are better arrays for storing the coordinates, e.g.:

cart_coord = flex.vec3_double(flex.grid(n_real))
p_weight = flex.double(rmap.size())

for plain_ind,grid_point in enumerate(flex.nested_loop(n_real)):
  cart_coord[grid_point] = grid2cart(grid_point)
  p_weight[plain_ind] = rmap[grid_point]

> By the way, I noticed that many weights are negative, is this possible in a patterson map or am I doing something wrong?

This is normal - I believe it's a Fourier series termination error,
but I still don't fully understand the reason.  An Fobs map will have
the same problem; the mean will always be approximately zero.
Technically, if the map were perfect with no missing data, it would be
uniformly positive, but that's never actually the case.  There are
ways to "fix" the map so it's all-positive, such as maximum entropy
cleanup, but in practice I think we just ignore the negative values
(at least for Fobs/2mFo-DFc maps - not sure how Patterson maps are
usually handled).

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