Subject: Re: Problem with HyperNMR

From: Adnan Hazar <[email-masked]>
Date: Thu, 28 Aug 1997 20:12:56 +1000 (EST)

Sometime ago I presented a problem I had with HyperNMR.  It just was not
able to deal with "large" no of protons. Including from HyperChem I have
received the following messages:
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From: Wayne Steinmetz <[email-masked]> 

One eventually runs out of memory and then has to be sly and crafty.
I face the problem with erythromycin.  Fortunately, more of the spins are
weakly coupled to one another.  In order to handle the portion of the
spectrum due to the strongly coupled spins, I consider a subspectrum.
For example, in the desosamine ring attached to the macrolide ring of
erythromycin, the 2' and 3' protons are nearly degenerate so strong
coupling is assured.  The 2' proton is coupled to the 3' and also a 1'.
The 3' is coupled to the 2' and also to the 4'axial and 4'equatorial.
The network goes on but
to handle the portion of the spectrum due to the 2' and 3' and perform a
simulation involving the 1', 2', 3', 4'equatorial, and 4'axial protons.
Normally the total spin system of a molecule can be broken down to
subsystems, manageable islands of spins.   

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From: Serge Pachkovsky <[email-masked]>

This looks very much like an excuse by a sub-standard support droid.
There indeed are quite a few spin states in this system. (Not 2^37,
however. More like 2^37 * 2^17 * 6^3 * 3^7, if you assume the molecule
to consist entirely of H-1, C-13, O-17, and N-14) The nice part, though,
is what THIS IS ABSOLUTELY IRRELEVANT.

Depending on gory implementation details, a strightforward realization
of GIAO-SCF shielding tensors at the INDO level (which, I believe, is
the level employed by HyperNMR) may require from about 12*Norb^2 (if
the developer in question feels generous) to (9*Natom + 3)*Norb^2 (if
he insists on been wasteful) words of memory. In fact, it is possible
to go by with much less than this. Coupling constants are a bit more
difficult, with something between (3*Natom + 9)*Norb^2 to 12*Natom*Norb^2
for a naive implementation. I do not know enough about HyperNMR to be
able to guess how sofisticated its memory handling is.  

In your particular case, there are 64 atoms and 145 orbitals, so the
shilding tensor calculation should need from 2Mb (for a "generous"
implementation) to 93Mb (for a "wasteful" one). Coupling constants
might need from 32Mb to 132Mb, possibly in addition to the shielding
tensor requirements. Keep in mind what these are back-of-the-envelope
estimations, actual requirements may differ by a factor of 2 or four
in either direction, depending on the actual implementations' quality.
Your support contact should be able to provide you with a tighter
estimate.

In any event, you should be able to compute shielding tensors for a
molecule of this size with 64Mb of real memory, provided that the
program and operating system you are using do not impose some arbitrary
limit on the resources which can be used. Coupling constants may be a
bit more problematic, though, both in terms of memory use and calculation
time.

Regards,            

/Serge.P   
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From: Joel Polowin <[email-masked]>

Hardware level: nothing you can do; that kind of memory simply
can't be packed into a PC.

Software level: You may be able to reduce the number of spin
states by marking some protons as rotationally equivalent, such
as the three protons on a methyl group.  (Use with caution: for
example, the six protons on two different methyl groups are not
necessarily all equivalent to each other!)  Apart from that, your
best bet is probably to calculate the spectrum for only part of
the system at a time and then take the total spectrum as being the
sum of the spectra of the parts.

Joel Polowin
[email-masked]       
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**********************
Adnan Hazar
Chemical Sciences
Victoria University
Australia
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