Major changes to Hydra introduced in Hydra4.0

New data format
The data files have changed format. Gas properties are now defined only for gas particles; the gas particles must precede dark matter particles in the arrays and the order of the arrays has changed. The new data format is described in the DOCUMENTATION. A conversion program to change old-style to new-style data formats is provided (see UTILITIES).

New units
The i/o units for non-expanding simulations have changed. The positions are now given in length units, as specified in inunit.F (they used to be in units of the box100). The boxsize and all cosmological parameters are now ignored.
The input positions for isolated simulations need no longer be in the range [0,1). It is possible to (optionally) specify an outer cut-off radius from the centre of the particle distribution, beyond which particles are ignored.
If you do non-expanding, periodic boxes, the the positions need to be in the range [0,1), but box100 is no longer used to fix the units; hence the boxsize must be specified as lunit.

New program structure
There are numerous changes. The most important one is that the gravity and SPH accelerations are now separated from one another. This makes the code run MUCH faster in gravity-only mode than before. For most applications, the SPH also seems to show a speed-up; however this is yet to tested on a wide range of data-sets. There remains a lot of optimisation to be done for the SPH.

Minor changes introduced in Hydra4.1

Modifications to the SPH

Hydra4.0 contained a rarely-triggered bug in that epp (the particle specific energy) was never allowed to drop below 1e-10 in shsph. This has been corrected.
A new temporary array has been introduced to store the old values of the density in the sph routines. This makes no difference unless the -DWEIGHTED_NEIGHBOUR flag is set when makes the h-update alorithm independent of the order that the particles are scanned.
The artificial viscosity force has been modified to make it symmetric in specific energy between a particle and its neighbours. This reverts back to a form of artificial viscosity very similar to that in Hydra3.
The change in energy due to artificial viscosity has been altered to use comoving relative velocities rather than physical ones. If this seems odd then look at the comments in sph.F.

New -DBYTE_SWAP option
If -DBYTE_SWAP is set in makeflags then Hydra will convert to/from little-endian on input/output, storing the data files in the opposite endianness to the internal representation. This allows easy transfer of data files between different machine architectures. i/o is a little slower so if only one architecture is to be used then leave this option unset.

New -DCPUTIME option
The cputime used is no longer automatically sent to stdout but will be if the -DCPUTIME option is set.

CMBFAST transfer functions
There is a new option in cosmic to read the transfer function from a CMBFAST output file (see COSMIC).

Minor changes introduced in Hydra4.2

Change to the SPH
The SPH PdV work term is modified by a factor, fhvar, to correct for the spatial variation of the smoothing kernel, as described in Springel & Hernquist (astro-ph/0111016). This vastly improves entropy conservation. It can be switched off using the -HNOVAR flag.

New -DEXACTNSPH option
New -DEXACTNSPH flag to force the SPH routines to do a partial sort to find exactly nsph neighbours. Surprisingly, this does not seem to slow the code down.

New -DENTROPY option
Exact entropy conservation from PdV work can be ensured by using the -DENTROPY flag. This leads to acceptable (but not fantastic) energy conservation. It is possible that the implementation of this could be improved and if anyone works out how then please let me know (p.a.thomas@sussex.ac.uk).

New -DPHOTO_SS option
This flag implements a steady-state ionization field modelled on that in Haardt & Madau 1996. It essentially limits particle temperatures to be hotter than about 10^4K (with the precise value depending upon density and redshift).

New -DSAVE_ACCEL option
New SAVE_ACCEL option to save the accelerations in dump files in order that restarts do not have to re-evaluate them. This both saves time and ensures that restarts give identical results (without this option there is an extra evaluation step).

-DRELAX option reimplemeted
The -DRELAX option to create a glass-like distribution of particles has been reimplemented.