Subject: CCL:Heat capacities with g03

From: Matthias Gottschalk <gottschalk=at=gfz-potsdam.de>
Date: Tue, 20 Apr 2004 07:47:22 +0200

Dear CCL,

as a naive geoscientist I try to calculate the heat capacities of 
specific single
molecules using g03 with something like that for Si2O7H6:

%chk=si2ooh6.chk
%mem=2048MB
%nproc=1
# opt=(restart, verytight) Int=UltraFine freq pbepbe/6-311++g(3d,2p)
geom=connectivity temperature=298.15 pressure=0.9869233
.
.
.
.

Everything runs fine. At the end I do get the following warnings within 
my results:

  -------------------
  - Thermochemistry -
  -------------------
  Temperature   298.150 Kelvin.  Pressure    .98700 Atm.
  Molecular mass:   173.96521 amu.
  Principal axes and moments of inertia in atomic units:
                            1         2         3
      EIGENVALUES --   890.447432428.485422488.20921
            X            -.63183    .74944    .19780
            Y            -.05907   -.30101    .95179
            Z             .77285    .58968    .23446
  This molecule is an asymmetric top.
  Rotational symmetry number  1.
  Warning -- assumption of classical behavior for rotation
            may cause significant error
  Rotational temperatures (Kelvin)       .09727      .03567      .03481
  Rotational constants (GHZ):           2.02678      .74316      .72532
  Zero-point vibrational energy     237943.2 (Joules/Mol)
                                    56.86979 (Kcal/Mol)
  Warning -- explicit consideration of  20 degrees of freedom as
            vibrations may cause significant error
  Vibrational temperatures:     22.32    47.39   164.92   198.54   252.18
           (Kelvin)            274.49   292.94   351.57   352.45   378.41
                               424.27   433.89   483.89   496.09   507.09
                               529.63   559.68   590.08   792.66   885.13
                              1071.96  1143.85  1168.05  1211.52  1224.45
                              1259.83  1274.99  1281.78  1340.88  1371.30
                              1389.84  1430.93  1501.79  5248.42  5427.26
                              5454.86  5462.94  5466.36  5467.29

  Zero-point correction=                            .090628 
(Hartree/Particle)
  Thermal correction to Energy=                     .103807
  Thermal correction to Enthalpy=                   .104751
  Thermal correction to Gibbs Free Energy=          .050334
                      E (Thermal)             CV                S
                       KCal/Mol        Cal/Mol-Kelvin    Cal/Mol-Kelvin
  Total                   65.140             46.134            114.529
  Electronic                .000               .000               .000
  Translational             .889              2.981             41.394
  Rotational                .889              2.981             30.066
  Vibrational             63.362             40.173             43.069
  Vibration  1              .593              1.986              7.139
  Vibration  2              .594              1.983              5.644
  Vibration  3              .608              1.937              3.189
  Vibration  4              .614              1.915              2.832
  Vibration  5              .627              1.873              2.378
  Vibration  6              .634              1.853              2.220
  Vibration  7              .639              1.835              2.100
  Vibration  8              .660              1.772              1.771
  Vibration  9              .660              1.771              1.767
  Vibration 10              .670              1.741              1.642
  Vibration 11              .689              1.683              1.446
  Vibration 12              .694              1.671              1.408
  Vibration 13              .717              1.603              1.230
  Vibration 14              .723              1.586              1.190
  Vibration 15              .729              1.570              1.155
  Vibration 16              .741              1.538              1.088
  Vibration 17              .757              1.494              1.004
  Vibration 18              .774              1.448               .926
  Vibration 19              .906              1.138               .542
  Vibration 20              .975              1.000               .424

In this regard I do have the following questions:

1. Some rotations might be frozen at low temperatures, but the 
rotational temperatures are here
so low that I can ignore the first warning at ambient temperatures and 
above. Right?

2. Most if not all of the vibrational temperatures are significant. The 
calculated total heat capacity
is too high because not all vibrations are active at the chosen 
temperature. Right? What can I do
to get reliable heat capacities.

Are there other options in g03?
How to choose the right vibrational contributions to cv?
Can I reduce the explicit consideration of  20 degrees of freedom?
Where can I learn more how to deal with the problem practically?

Any help is welcome.

Matthias

-- 

PD Dr. Matthias Gottschalk
GeoForschungsZentrum
Projektbereich 4.1
Telegrafenberg
14473 Potsdam
Germany

tel/fax  +49 (0) 331 288-1418/1402