You can refer to PNNL Compedium for clues. See the link.

http://www.pnnl.gov/main/publications/external/technical_reports/PNNL-15870Rev1.pdf

thank you very much Mark 

c Density = -0.001205

m1 &                   $ Air (C:-0.000124, N:-0.755268, O:-0.231781, Ar:-0.012827) 

6000.      -0.000124 &

7014.      -0.752290 &

7015.     -0.002977 &

8016.     -0.231153 &

8017.     -0.000094 &

8018.     -0.000535 &

18000.   -0.012827

Up to your x-section library (for example ENDF6) you can use a simple mixture model like:

m1 7014.42c -0.755 $ Air 0.001205 g/cm3

8016.42c -0.232

18000.42c -0.013

the standard composition and density of about 300 most used material for MCNP could be found in the  following link::

http://www.pnnl.gov/main/publications/external/technical_reports/PNNL-15870Rev1.pdf

all the best,

yaser

c

c Air with 1% water:

M2 1001 -0.00111967185672092

1002 -3.58077068509439E-7

2003 -7.01300681482183E-13

2004 -6.93002257732448E-7

6000 -1.269899924930E-4

7014 -0.744226018468389

7015 -0.002930937363427

8016 -0.238274132603651

8017 -6.338532733037E-4

10020 -1.188009929771E-5

18036 -3.8063224269139E-5

18038 -7.57346759599989E-6

18040 -0.0126264625590237

36078 -9.80896820559013E-9

36080 -6.47380864579173E-8

36082 -3.3656973211545E-7

36083 -3.37936500433013E-7

36084 -1.69483116165613E-6

36086 -5.26125375559769E-7

54124 -3.55783232684075E-10

54126 -3.37976986796745E-10

54128 -7.36915932270391E-9

54129 -1.02645376834607E-7

54130 -1.59506563063874E-8

54131 -8.38324527235628E-8

54132 -1.07054801524348E-7

54134 -4.21479276142738E-8

54136 -3.63078420449797E-8

MT2 LWTR.20T

c

c

c US-NRC REGULATORY CONCRETE, Specification for Neutron Runs:

M3 1001 -0.00999680296876265

1002 -3.19703123735226e-006

8016 -0.530588536334774

8017 -1.4114637E-03

11023 -0.029

13027 -0.034

14028 -0.309613753235242

14029 -0.0162829984920268

14030 -0.0111032482727315

20040 -0.0425311889758111

20042 -0.000298038140943721

20043 -6.36695766319752e-005

20044 -0.00100663896675447

20046 -2.01802598486226e-006

20048 -9.8446313873892e-005

26054 -0.000790378165836526

26056 -0.0128662142579724

26057 -0.000302465885119295

26058 -4.09416910718163e-005

MT3 LWTR.20T Fe56.22T

c

c

c Mild Steel Composition Template for MCNP (MODE P E N)

M4 6000 -0.1700

14028 -0.00229683793201218

14029 -0.000120793757359249

14030 -8.23683106285717e-005

15031 -0.0003

16032 -0.000284305832423451

16033 -2.30964581360332e-006

16034 -1.33354572036474e-005

16036 -4.90645592983692e-008

25055 -0.008

26054 -0.0557329518081299

26056 -0.90725190824788

26057 -0.0213281658421263

26058 -0.00288697410186408

MT4 Fe56.22T

c

c

c Water:

M5 1001 +0.66656

1002 +0.00010666666666667

8016 +0.3325402

8017 +7.931333333333E-4

MT5 LWTR.20T

c

c

c Tissue Equivalent Material (average for entire human body; WIKIPEDIA)

M6 8016 -6.5414E+01 $ O in TEM

6000 -1.8115E+01 $ C in TEM

1001 -1.0064E+01 $ H in TEM

7014 -3.0191E+00 $ N in TEM

20000 -1.4089E+00 $ Ca in TEM

15031 -1.1070E+00 $ P in TEM

19000 -2.5159E-01 $ K in TEM

16000 -2.5159E-01 $ S in TEM

11023 -1.5095E-01 $ Na in TEM

17000 -1.5095E-01 $ Cl in TEM

12000 -5.0318E-02 $ Mg in TEM

26000 -6.0382E-03 $ Fe in TEM

30000 -3.2204E-03 $ Zn in TEM

14000 -2.0127E-03 $ Si in TEM

40000 -6.0382E-04 $ Zr in TEM

82000 -1.7108E-04 $ Pb in TEM

29000 -1.0064E-04 $ Cu in TEM

48000 -7.2458E-05 $ Cd in TEM

23000 -2.6165E-05 $ V in TEM

50000 -2.4153E-05 $ Sn in TEM

80000 -1.9121E-05 $ Hg in TEM

28000 -1.4089E-05 $ Ni in TEM

22000 -1.3083E-05 $ Ti in TEM

42000 -1.3083E-05 $ Mo in TEM

51000 -1.1070E-05 $ Sb in TEM

24000 -2.4153E-06 $ Cr in TEM

47000 -1.0064E-06 $ Ag in TEM

92238 -1.3083E-07 $ U in TEM

4009 -5.0318E-09 $ Be in TEM

MT6 LWTR.20T

c