# The Siesta FDF input file¶

The main input for Siesta is a text file in FDF format, which contains a description of the system to be simulated, a (maybe implicit) script of actions to be taken, and a specification of the quality parameters of the simulation.

The FDF format allows data to be given in any order, or to be omitted in favor of default values. Here we offer a glimpse of it through the following rules:

• The basic syntax is a ’data label’ followed by its value. Values that are not specified in the datafile are assigned a default value.

• Labels are case insensitive, and the three characters - _ . (hyphen, underscore, dot) in a data label are ignored. Thus, LatticeConstant, lattice-constant and LAttice_ConstaNT represent the same label.

• All text following the # character is taken as comment.

• Logical values can be specified as T, true, .true., yes, F, false, .false., no. Blank is also equivalent to true.

• Character strings should not be in apostrophes.

• Real values which represent a physical magnitude must be followed by its units (see this section).

• In some cases it is important to include a decimal point in a real number to distinguish it from an integer, in order to prevent ambiguities when mixing the types on the same input line.

• Complex data structures are called blocks and are placed between %block label and a %endblock label.

• You may ‘include’ other FDF files and redirect the search for a particular data label to another file. If a data label appears more than once, its first appearance is used.

• If the same label is specified several times, the first one takes precedence, since the FDF parser stops looking for a label when it finds it.

• If a label is misspelled it will not be recognized (there is no internal list of “accepted” tags in the program). You can check the actual value used by siesta by looking for the label in the output fdf.log file.

These are some examples:

SystemName      Water molecule  # This is a comment
SystemLabel     h2o
Spin polarized
SaveRho
NumberOfAtoms         64
LatticeConstant       5.42 Ang
%block LatticeVectors
1.000  0.000  0.000
0.000  1.000  0.000
0.000  0.000  1.000
%endblock LatticeVectors
KgridCutoff < BZ_sampling.fdf

# Reading the coordinates from a file
%block AtomicCoordinatesAndAtomicSpecies < coordinates.data

# Even reading more FDF information from somewhere else
%include mydefaults.fdf


The file fdf-XXXX.log (with XXXX standing for some process-dependent numbers) contains all the parameters used by the program in a given run, both those specified in the input FDF file and those taken by default. They are written in fdf format, so that you may reuse them as input directly. Input data blocks are copied to the fdf.log file only if you specify the dump option for them.

Magnitude

Unit name

MKS value

mass

kg

1.E0

mass

g

1.E-3

mass

amu

1.66054E-27

length

m

1.E0

length

cm

1.E-2

length

nm

1.E-9

length

Ang

1.E-10

length

Bohr

0.529177E-10

time

s

1.E0

time

fs

1.E-15

time

ps

1.E-12

time

ns

1.E-9

time

mins

60.E0

time

hours

3.6E3

time

days

8.64E4

energy

J

1.E0

energy

erg

1.E-7

energy

eV

1.60219E-19

energy

meV

1.60219E-22

energy

Ry

2.17991E-18

energy

mRy

2.17991E-21

energy

Hartree

4.35982E-18

energy

Ha

4.35982E-18

energy

K

1.38066E-23

energy

kcal/mol

6.94780E-21

energy

mHartree

4.35982E-21

energy

mHa

4.35982E-21

energy

kJ/mol

1.6606E-21

energy

Hz

6.6262E-34

energy

THz

6.6262E-22

energy

cm-1

1.986E-23

energy

cm**-1

1.986E-23

energy

cm

1.986E-23

force

N

1.E0

force

eV/Ang

1.60219E-9

force

Ry/Bohr

4.11943E-8

Magnitude

Unit name

MKS value

pressure

Pa

1.E0

pressure

MPa

1.E6

pressure

GPa

1.E9

pressure

atm

1.01325E5

pressure

bar

1.E5

pressure

Kbar

1.E8

pressure

Mbar

1.E11

pressure

Ry/Bohr**3

1.47108E13

pressure

eV/Ang**3

1.60219E11

charge

C

1.E0

charge

e

1.602177E-19

dipole

C*m

1.E0

dipole

D

3.33564E-30

dipole

debye

3.33564E-30

dipole

e*Bohr

8.47835E-30

dipole

e*Ang

1.602177E-29

MomInert

Kg*m**2

1.E0

MomInert

Ry*fs**2

2.17991E-48

Efield

V/m

1.E0

Efield

V/nm

1.E9

Efield

V/Ang

1.E10

Efield

V/Bohr

1.8897268E10

Efield

Ry/Bohr/e

2.5711273E11

Efield

Har/Bohr/e

5.1422546E11

Efield

Ha/Bohr/e

5.1422546E11

angle

deg

1.d0

angle

5.72957795E1

torque

eV/deg

1.E0

torque

1.745533E-2

torque

Ry/deg

13.6058E0

torque

0.237466E0

torque

meV/deg

1.E-3

torque

1.745533E-5

torque

mRy/deg

13.6058E-3

torque